My bassani x-pipe w/ cats was advertised to increase my HP by 30, but that's nowhere near true. There were significant gains, but definately not 30! I mean, the effects it had were
1. it made my borla cat-back sound gorgeous!
2. I lost some low-end torque, but gained a good amount in the higher RPM's
3. My gas mileage improved (slightly, and only when I drove nice)
I mean, if you like the way your MAC cat-back sounds now, you'll really like it if you get the x-pipe. Since you've got 3.73's then you shouldn't suffer from a loss of low-end torque. You'll probably notice a couple tenths drop in you 1/4 mile time with your current set-up
2000 automatic gt-
borla cat-back, bassani x-pipe w/ cats, steeda MIL's, k&n fipkit, cobra calipers up front, wolf springs, KYB adjustable shocks, C&L intake plenum, MAC 70mm throttlebody, kenny brown subframes w/ cross brace, plasma booster, SLP 180 degree thermo, frpp 3.73's, superchip custom tune
new #'s - 241 rwhp, 290 rwtq