I have a cutout on my 98 SS/T. Sounds badass. There's no performance gain or loss thats noticeable. No mpg gain or loss noticable. Its strictly to scare other cars around you.
I passed emissions in WA state, and I feel as though its pretty strict. I open it on occassion and PD doesn't care too much, just don't get WOT and it stays pretty quiet.
Here's my cutout
and a video of it open going 120+ mph.