I go to www.mattrandolph.com and click on the WRX link at the bottom of the left side of the page. You may have to change your text size if you can't see it since there's no scroll bar. This guy has compiled one of the most helpful pages when it comes to researching exhausts.

 

Go to turbo magazine and check out the exaust shootout they did for the wrx. They tested some 15 systems and gave $, hp gain over stock and tork gain over stock. Very helpfull. Here is my 2 cents. Love the new sound, good deep rumbel, loss of some low end tork (1000-2000) rpm, I haven't yet got the comp reflash. Gain in tork and HP above 3000. You should here it up around 6500 r's

 

i use to deal with m2 (formally mostley mazda) when i had a RX7. their stuff is quality stuff. i dont know much about their wrx stuff tho. but their hp to $$ ratio isnt too good.

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I like and will probably be getting the Cobb Tuning Catback, it doesn't look like a melon shooter (90mm as aposed to 120mm) and isn't quite as loud but still sounds great. I wish it wasn't so heavy but it's also a lot cheaper than the some of the others. If you have the money though I've heard nothing but good things about the HKS Carbon Titanium.