Quite the opposite, for small (175hp 79 jet to 225hp 82 jet) shots the motor GAINED over 250hp and 300hp.

Before the nitrous pulls, we retarded the timing to 30deg advance and made a pull. The motor on retarded timing made 650HP.
If we weren't worry about pushing the crank though the
2 bolt mains in the dyno room, It is very possible the motor could make over 1000HP. My engine builder didn't want to blow up the motor on the dyno and wanted me to play with the motor for a while before we really lean on it. We shot the nitrous at 6000RPM on the dyno.
The over achieving gain is mostly the camshaft. It was designed by a nitrous guru, but it kills power on the motor if you can believe that.
The headers came from the Fabshop. The guy mostly does puller type headers but he can make the headers for drag racing. He has a jig for pickups!
The header he made me have 2.25 primaries and shorter than his truck pulling headers.
http://www.fabshopheaders.com/categories/4x4-headers-1.html