i just installed the trubendz borla 2.5" on my ride. it really sounds good! its got a low pitch to it, not ricey at all. this is what i did:
first, i put the cat-back on. i didnt put the flexpipe on yet b/c i ordered the svt header and in my case i went with the svt flex b/c i found a good deal on a svt cat. so, i had to wait to put the flex on b/c it wouldnt fit with my stock header/cat.
if you do put a flex on with your catback, i would make sure that it will bolt up to the header/cat you plan to get. you dont want to have to buy another flexpipe so you can put your header on.