A few years back I had a monster type IV motor built for my 914 at Fat Performance and their machinist also did the heads. I was amazed by work on the heads. they were completely welded up and remachined to accept 48X38 valves and the combustion chamber, intake and exhaust ports were all highly modified. They flowed unbelievably well. I have never seen this done with 911 heads but if they turned out like my type IV heads I want some.

Looks like the heads Buell uses on their bikes. Very similar design. I always wondered why Porsche didn't do a better head design. The Hemi is decent for air, but awful for thermal efficiency.

Here is another take on the modified Hemi. Prostock Hemi Ford head.
http://www.theoldone.com/boss429head/SuperSwirl3.jpg