SCAT Enterprises of Redondo Beach has been producing some unique heads for Type I, II and III VW engines for a number of years. Instead of 2 heads, one for each bank of cylinders, SCAT heads come in sets of 4, like most modern aircraft aircooled engines.
These heads were used by HAPI Engines on their "Magnum" series, that claimed 75 HP with 1835cc, allegedy due to the high flow and volumetric efficiency of these heads. Rex's literature also touted complete immunity to a problem of dual port heads - cracking from the intake port to the spark plug hole. They also allowed a very neat, tidy, dual plug setup, with two 10mm plugs on the top of the head, where they are easy to get at. Mosler, and later TEC, the successor companies to HAPI, used the same heads.
Here is a shot of these heads on my engine:
Here is a comparable view of a conventional VW head, a CB Performance 044 big valve head:
Now look at the next picture. It shows a couple of display heads, that have been sawn down the middle of one combustion chamber. Both of these dual port heads have the exhaust port to the left. The head on the left is a stock 040, the head on the right is a CB Performance 044 big valve head. Note that both heads have extensive air passages and finning around the exhaust port. The big valve head has less fin area, and less air flow area in this critical region. For most racing applications, this isn't too big a deal, they are rarely required to put out power for more than 1/2 hour in road racing or circle track events, and only for about 12 seconds in drag races. The 044 head design will clearly produce more power than the stock heads at high RPM, with high lift cams, extractor exhausts, and huge carbs or injectors. None of these conditions apply to aircraft usage.
I don't have a comparable picture of the SCAT heads, but I just went and looked at them. The area from the intake port to the exhaust outlet is solid, I.E. there is NO place where you can see through the head, and therefore no place to force air around either port for cooling. There are fins, mostly on the bottom, and a few short ones on the top of the exhaust outlet, which you can see in the first picture.
Another thing: There are 8 fins on the main portion of the stock head. The SCAT heads have 5.
For a 1/2 VW application with the heads out in the airflow, I think they'll be fine. But in my tightly cowled, downdraft cooled SONERAI, I'm a little concerned. Ed Sterba, propmaker and SONERAI guru, emphasized the importance of forcing air through the slots in the stock heads, his CHT's and valve life got into the acceptable range only when he devised baffles that did this.
For some other, mostly positive, opinions on SCAT heads for aeroengines, try these mailing list archive links, and this web page.