Headers tend to be pretty weak for headers. Around a 10bhp gain. More for the better ones.
What they *are* good at, is being interesting when you run a S/C. The tiny M45 the TRD S/C is based around... The anemic gains from the headers actually nearly exactly matches the delta power curve of the M45 S/C. So basically, you loose power to parasidic drag, then gain the same power band with headers.

I just find it interesting how that works out.