Horse Power Potential of Cylinder Heads

Dave - do you have Larry Meaux's Pipemax program? You're obviously pretty interested in the specifics... It's a fun program to play with and might help you with the limited intake rules and things like shift points and final gearing choices for various tracks, and maximizing your exhaust system.
In terms of frictional loss the biggest is the ring package you run. Low tension is good in terms of the oil ring, but the 2nd ring (scraper) should be a Napier, and all three can be reduced in width. (I'm also assuming you can't run a vacuum pump?) There are big gains there.