Does your SE have the 3.8 or 3.4? I had a 3.4 in my Monte Carlo LS and did a custom cat back with u-bend delete, 2.5" tubing and dynomax ultra welded. The car sounded great, but I lost torque down low. Especially right where I cruised at highway speeds. My car had the 2.86 final drive, so I was taching under 2000 at 70 MPG. When using A|C or in a hilly area, my car was constantly in and out of OD. It was annoying.
_________________________
03 GT, STB, GM HD sway bars, 35% tint, GE893(37watt)fogs, Flowtech Terminators, 180 T-stat, ZZP DP & PCM, Ceramic PLOG & Crossover, 18" glasspack, 2007 17" Mustang Bullits 235/55/17 Pirelli PZero Nero, 12" R1 D\S rotors, gutted air box, GTP radiator