Just a Quick little question id like to add:
Lets say you changed the internals (piston/cams/rods/ect) would you be better off with a custom designed* header than lets say a 'tuner' header?

*Designed and not 'made' for a reason. Lets say you did some testing and calculations aswell rather than 'lets do a 4-2-1 with nice looking bends' :P