I asked Brenspeed, they tested it on their GT, and say :
Torque at wide open is not affected. Just some cars at part throttle will experience a loss in low end but not at wide open throttle. They do not cause noises, vibrations, or anything that customers have not liked. Most cars pick up around 6 rwhp and for the money that is a good gain.
2006 Vista Blue Mustang GT: Mods: ||EdelBrock E-Force Stage 1// Brenspeed 93 Dyno Tune// Pypes LT headers, O/R H-pipe// GT500 Fuel System// Borla Stingers// MGW Shifter// Hawk HPS// Powerslot Rotors// CHE: UCA / LCA/ Anti-Squats/ K-member/ Panhard Brace// Nitto NT555's (daily)// Infinity Sound// CDC Chin// MT ET Streets (track)||
447rwhp 419ft-lbs - Stage Zero
* 466 rwhp 441ft-lbs - Stage 1 *