I think that subaru is pretty bogus when it comes to stuff like this. STi stand for: Subaru Technica International. I'm assuming that Subaru (Fuji Heavy Industries) is in kahoots with STi. I just don't understand why they would pull something like this--not supporting their own F'IN products.
what you guys should do is just ask your dealer before doing anythign to your car. Chances are that they're going to give you an answer liek "well if the problem is caused by what you did, then it won't be covered under warranty."
For example, if you get a boost controller and you messed up your turbo, engine, or transmission and there is proof that it was caused by the mod. you're at fault.
All that stuff is very vague but a simple mod like an exhaust shoulnd't cause any problems and it shouldn't void any warranties.
I think as far as North American STi parts go,(and yes there is a difference between NA STi and the rest of the world) you really have to look at the cost Vs the effect. The STi Sport Muffler is WAY expensive with mediocre at best effect. Really all it's doing is changing the tone of the exhaust. The reason they provide the disclaimer is more a matter of noise decible then emmissins or performance. I mean what can Subaru do if we get noise violation tickets w/ their muffler? For the same money or less you can get a cat-back exhaust that will provide much greater effect then the STi muffler. Or for a few hundred bucks more a completely new exhaust system from a reputible manufacturer and get great results. If you are a member of i-club do a search under turbo powertrain for Brullen Exhaust posted by GTBGuy. It's a very interesting exhaust and with a bit more info I may get one.
The "STI" axel back eghst that is sold though the dealer is made by brospeed it is not a STI part. Now I may be wrong.....(it would not be the 1 st) I know the dealer may have said it is STI but there was a thead on the I club about this. It would also explain why there is no STI logo on the tip or anywere were else on the can for that matter.
I have initiated a "claim as it were with Subaru of america just so you guys know the only way your warranty is void is if you race your car. PERIOD. Any mods no mods it doesn't matter. I have the STi exhaust (and im selling mine.) it sounds great and my dealer in town thinks so as well. Your also correct about the Direct Effect theory. you put it on and that specific part breaks something that it touches or effects and youre void.