About Calver ST heads 

Terminology - 
VFM - Value For Money 
CR - Compression Ratio 
VE - Volumetric Efficiency 
'Modified to give maximum performance gain for cost. Combustion chambers, inlet and exhaust ports extensively re-worked. Smoothed finish in ports - not mirror-finish polished - to minimise intake charge drag. Multi-angle valve seats in head. Cooper S size valves with current maximum flow profiles and plasma-nitrided for durability/longevity particularly when used with unleaded fuel. Combustion chamber volume used to give slight static compression ratio increase over standard. Head mods employed greatly increase volumetric efficiency, boosting actual running (dynamic) compression ratio. Where much sportier cam profiles are used, a higher compression ratio is advised to achieve maximum power potential. Seek advice on what is required'. 
That is the opening paragraph from the fitting instructions sheet as supplied with the heads I do. A very short, what's involved report and is a pretty accurate description. After all - what else can you say about a modified cylinder head? For some the 'visual' is the only way to go - the shinier the better in most instances. The better a head looks, the more power it'll give. Absolute hogwash. 
Oh, I don't mean everything that glitters isn't gold; there are one or two A-series head specialists out there that produce what are essentially absolutely sublime works of art that deliver the goods too. But just like art - the more desirable the higher the price. Some have knocked me for my relatively 'plain' looking heads, saying they don't look anything special. Some saying they bought A.N.Other's head because it was 'better finished' (i.e. polished to a mirror finish). Well this ain't a fashion show guys. I've recently been asked to 'justify' my heads by prospective purchasers. Right off the cuff my initial reaction is 'why should I? I know they work'. But then that's just the problem isn't it - I know, but how on earth do you folks know? Excepting word of mouth by those that have bought and used my heads. 
I have done tests in the past where I've dynoed an engine then tried several different aficionados 'off-the-shelf' heads on it so see what gives. But then to name names would start the mud slinging - and I'm not into that, and it wouldn't be fair without the other side to fight their corner. It would probably be labeled as a 'fix' as I wasn't an independent tester. But then again - how do these other aficionados know how their heads compare with others if they don't do this? Perhaps the smuggest of them just think their heads just can't be bettered, others perhaps pandering to the customers desires at that period (38mm inlet valves, inclined heads, 'tunnel' ports, high peak airflow figures, etc.). And who can blame them? After all - they're giving you what you want, right? Well, I've never pandered to general public opinion. I've always turned out what I believe to be what's good - not guessed at but well tried and tested and from a continual R&D program. Especially where VFM is concerned. Like I don't get carried away mirror-polishing every nook and cranny. I've never found any performance gain, it takes time and extra materials to do it (which someone has to pay for), gets dirty the first time you fire it up, and you can't see any of it once it's fitted to an engine. A layer of carbon is a far better heat insulator than a mirror-polished chamber that lasts 60 seconds. That does not constitute VFM as far as I'm concerned. 
Rather than do a 'mine's better than his' type of deal - which is what I think a number of folk would like to see - I'll just do a 'what I do and why I do it' thing. Perhaps it'll help. 
So cutting the opening paragraph down into it's component parts, we start with 'modified to give maximum performance gain for cost'. Exactly what I've just been banging on about. To me, there is no point in doing something that takes time and materials that doesn't help the cause in achieving the ultimate goal - more power but at reasonable expense. If you're not fitting a modified head for this reason, stop reading and buy the prettiest head you can find for whatever you have to spend/can afford, as none of the following will make an ounce of difference to you. 
'Combustion chambers, inlet and exhaust ports extensively reworked'. And so they are, but not over the top. Over the many years I've been modifying heads, I've distilled out what are potentially the most important aspects to give the kind of airflow in the right places that affect different driving disciplines. There is absolutely no point whatsoever to providing a lusciously modified head that gives massive peak airflow figures when that peak is at 0.500"-0.550", yet the discipline only calls for 0.400" lift. Believe me when I tell you I've flow tested LOTS of heads from other experts where they achieve promised peak flow figures that would leave you dead in the water with the poor figures they get up to 0.350" lift. Not to mention the gross inconsistencies port to port. We're talking area under the curve here - and that's what I try to achieve. For a street car, you need a head that'll flow well in the most used range of lift that gives the best performance for that 'discipline'. As far as potential over-all performance then - VFM. 
Smoothed finish in ports - not mirror-finish polished - to minimise intake charge drag. Yes it does. Fuel can stick to smooth shiny surfaces, reducing fuel efficiency and economy - particularly where overly large ports are used which reduce gas speed within the port and are highly polished . On a racer that is driven flat out everywhere perhaps this isn't such a problem. And highly polished ports will slightly increase airflow in certain instances by increasing boundary layer speed. So on a racer where port velocity is being driven hard, fuel fall-out is less likely. On the street, however, you certainly don't want fuel pooling in the intake ports and dribbling into the chambers when tottering about at low speed. If nothing else the excess fuel will do it's damnedest to extinguish the spark in the combustion chamber, drastically reducing torque. Let alone increased bore wear caused by bore-washing and horrendous fuel consumption figures. The smoothed finish is easier and quicker to achieve than the highly polished item. It also gives a better over-all port surface finish - i.e. reduced lumps and bumps that polishing simply won't remove as the polishing agent isn't resilient enough. Definitely a VFM point. 
'Multi-angle valve seats in head'. Again we have something distilled out over many years as giving the best all-round result. Not the highest flowing figures, not the thinnest 'pencil-line' seats, and not the very best of each angle - but a good, consistently well performing set of angles and dimensions that promote better than average air-flow that err on the side of longevity. Having to strip the head off your motor every 3,000 miles to refresh the valve seats/valves to maintain performance is a right pain. And costly. 
'…valves with current maximum flow profiles and plasma-nitrided for durability/longevity…'. For the full story on the valve profile deal - check out the relevant article in my 'Corner'. Just because certain well-known and respected A-series exponents sell a particular type of valve at a fairly stiff price doesn't mean to say they are the best. It's just what they do. 
Remaining is the combustion chamber size thing. This needs little expansion from what has been said. I use a fairly standard chamber profile because it is very hard to beat when used with the generally available/used piston crown configurations. Nothing exotic is required; the standard chamber shape gives very efficient burn characteristics. Worth noting is the bit about increases in VE giving rise to higher dynamic CRs. Some folk get obsessed with high static CRs. Going too high with an efficient head will cause detonation problems and power/torque loss through pumping losses. 
Remember my ruling - 'Tuning should be a compromise between what's possible and what's necessary'. Stick to this and you'll have years of hassle-free fun.