Those headers may be and probably are hurting you some. Whats the rest of the specs on the headers, pipe length and collector diameter and length.

As Poly suggested , you don't need much of a collector length with a stall of 7000 and a peak hp of 9000 rpm. I too want to know why you shift much lower then hp peak. I'm guessing you tried shifting at and above with no difference in ET???

Pipe size has a rule of thumb with exhaust port flow for port velocity for exhaust scavenging. I don't know what a w7 exhaust port flows. But in general, you size it from that. So whats your exhaust W7 ports flow?