Just because a head flows a bigger number, doesnt mean it will perform better. The peak number at .500 or .600 is meaningless. Add up the flow numbers from .100 up to .600, then divide by 6. Or, go in .050' increments and divide by 12. That average number is the only thing that matters. If the guy running the flow bench can block a small spot in the valve pocket with clay, and the head still flows the same number, then those heads have velocity issues. flow numbers are only part of the mystery. If the old turbo and cylinder head were complimenting each other well before, the new turbo may become a bottleneck with some mondo race heads.
Also, if you're filling the cylinders more, but then turning around and bleeding off a bunch of that air back into the intake, with a cam that closes the intake valve too late after bottom dead center, then there is no way a huge flowing head will help you. It starts to become a dance of, get more air in the cylinder, and keep it there.