Compression ratio vs Porting
High flowing heads are mostly helpful in the upper RPM range.
In general a 1 point bump in compression is 3-4% gain.
I believe cam and dynamic compression ratio plays a vital role here as well, generally 7.5-8.5 is the number to shoot for:
http://www.wallaceracing.com/dynamic-cr.php
It's all a system and one part works with the other to give you the end result.
PS: Get a good converter if your going automatic. :D