i have Bassani O/R X (catless) and SLP LM2 catback. I love it, noticable at idle but not crazy, and loud and proud when you get on it.
1998 Mustang GT, Bassani X-Pipe, SLP Loudmouth 2 Catback.
video clip of it on my stock motor before anything else really went on the car.
as long as you have cats on your midpipe, it will be street legal. the stock midpipe has 6 cats, but the O2 sensors are before the cats, and 2 more after the cats. and the guys with catless X's like myself, when you get a tune or tuner, you can turn off the rear (after cat) O2 sensors and it will not throw the light, but you will need a shady inspector to pass you. most average joes at inpsection shops just require a little monetary motivation, if you know what I mean.
and yes SLP tips are awesome, the came with my catback. you can tell a difference in the sound from the double walled SLP tips and regular tips. and they look awesome as well.