On a 715hp motor I had on the dyno, swapping from my dyno headers(2-2 1/8 x 4) to a set of 1 7/8 x 3.5 Hooker fenderwells cost about 40hp peak to peak, and 54hp at 6500rpm.

With the 1 7/8 headers installed in the car(678hp), at 3450- 3500lbs race weight, it went a best of 9.68 @ 137 in good air.......1.28 60'.

I've been talking with a guy recently who is working through a new combo.
4+ mph difference between two different converters, both custom built for the application, with no other changes to the car.
In his combo that shows up as a 65hp difference.