i dont know how much power you loose on a 4l80. I have a gt4276 on a stock ls1. 17 degrees of timing, 6 psi i made 442/467 floating valves. My power peaked at 5,000 rpms. With valve springs im guessing i will hit 470/490 @ 6 psi. Im on a mustang dyno with a t56, and have a 10 bolt with 4.10 gear(yea i know it sucks, 9 to be ordered soon)
torque looks great, but im not sure about hp.
MWC Fab 9/MWC Cage/MWC's First! Front End
LRE 427 LS Next/GT5533R/PRC 247cc/FLT TH400 XXXX whp @ XX Psi