Ok im considering going with a hks hiper for 475 cause it seemed like a good deal. Is this a good catback and what not? Also i was wondering it when i put on the exhaust if i should just do the headers and the UP/DP? Would it be detrimental if i did this without any engine managment. I realize i wont get the full potential but will anything serious happen or should i just reconsider my whole plan and possibly do Ap first? Can't decide what to do any help would be appreciated. Oh ya Helix UP DP dont know bout the headers any suggestions?
I don't know a lot about aftermarket headers, but from reading the forums on this site i hear they are a waste of money. The stock ones will do unless you plan on seriously modding your car, although they do add to the sound of your exhaust. If you get equal length headers it will take away that famous Subaru boxer engine sound. I am currently running a turbo back exhaust and up pipe w/o engine management and haven't had any problems.
Up/Downpipe, Catback, and Cobb AP is your best bet. As far as how good it is. Performance wise they are all very close, within a few HP. If you want to hear it, try looking up "hks hyper wrx" on youtube.com or streetfire.net.