When i had my 3 in downpipe an header install i noticed a night an day difference really opened that thing up. Maybe it could be that u losing a little bit of low end torque replacing it with more high end torque? All depends on at what point u usually step on it. I noticed a gain because i always drive on the highway so i feel it alot but if u drive on regular roads an press the gas u will notice that low end not picking up like it used too.