Dealers just say "abuse" and refuse to cover stuff. Warranties on sports cars are all bad. It doesn't matter how much it's modified. You can play the "it's stock, because I just put the stock stuff back on" game with them, it works sometimes. Generally, they'll decide if it's abuse or not independent of the exhaust mods on the car.
Everyone always says "void the warranty" like it's some kind of clause. It's not. They have to show that abuse or modification you have done has broken the part they would normally cover that they are denying. They are not going to deny service on your stereo or air conditioner because you have an exhaust on your car. They are not going to deny coverage of the window motors if you replace the turbo. You can't void shit.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD