With no cats you get good turbo spool up and wind down noises compared to stock. Sounds really good but stays quiet ever all.... I love my QTP downpipe with the electric cutout valve coupled with the stock cat back
WRX at 218 WHP (dynapack, aprx 275 BHP) and EVO at 375 WHP (Mustang dyno, aprx 510 BHP) on pump gas.
If you are on a budget you could spring for the ebay exhaust system. It think it's around $300 or so and includes an UP/DP and catback. Since you already have an UP you could just sell the UP that is included with the ebay package and recoup a few bucks. Several people on here have installed the system and have been happy with it so far. Click on the link below for the thread.