Get the downpipe and cat back together. Most exhausts are pretty simlar. Get 3 inch minimum. I use 80mm. I would also get an uppipe if you can. For maximum power go cat free but if you have emmissions there are a few systems that include a "high flow" cat (which there is no such thing).
Ge tthe whole setup if you have the dough, it should cost you about $1200. But if go for a catback for aorund $600 you have the extra $600 for a chip or a boost controller, and maybe even some springs to stiffen up the ride.
That is the best advice you can get..... period!
For me, I have a MRT cat-less and a up pipe. If you wan't to stay leagal though, this set up is not for you, go with a cat back. No real performance gain but it will sound better and look cool.