Vizard did a recent article for Pop hot rodding where he said anything over 2.2cfm per horsepower doesn't gain you anything (but extra weight).

Mainatining Velocity away from the exhaust valve and minimal downstram restrictions are at least AS important as OD. FAST guys are going high 9's with full dual 2 1/2" exhausts and Factory casting exhaust manifolds (obviously ported but still not as good as a race header)....that's pretty telling to me.