Midpipe Q?

It would sound better, but the stock mufflers should go too if you are getting a new midpipe. Just get 2 new mufflers put on at the same time too instead of paying 500 bucks for a whole new cat back. As far as X-pipes, SLP makes good hi-flow catted ones, and the design of the junction is similar to that of Dr. Gas, which is probably the best. I have SLP, and it fit real nice with good clearance and a killer sound.
 
It will sound much better. If you are really strapped for cash just get an offroad H, and switch out your stock h-pipe with that. Keep your stock cat-back.

Aftermarket cat-backs do almost nothing performance wise as opposed to changing out a midpipe with 4 or 6(cant remember) cats on it.

Do you think your car sounds good now, but needs more volume? O/R H is probably the best answer.