Finally got my Stillen Y pipe, gonna get it installed sometime this week...i'm still wondering if I should buy a hi-flow cat too? i'm just looking for performance, but even if it just adds like 3-5 hp, i'd really like to get more a difference difference by putting several mods on at the same time rather then doing it step by step

 

The high flow cat is good if you are going to install a supercharger otherwise don't bother. A cat-back exhaust
would work very well with the Y-pipe and intake. Y-pipe adds about 10-14hp and the full exhaust is 10-13hp.