Again, the onus is on them to provide an explanation of why the part you put on was the catalyst for the damage. Case in point, if you have an aftermarket downpipe on your STI and you grenade your engine, Subaru will tell you that the additional flow of exhaust through your downpipe caused an overboost condition that then blew the motor. They have said that to some people. You can potentially link that part with the failure.
I'm not saying that the dealer won't fight you, I am just saying that they would have to prove it if you had to go to court.