milltek do a cat back and a full decat, but the full decat will require a remap. so not a good option.
If I was you go and get the 340r pack fitted by Subaru then you know its covered under the warranty. If you put anything on it that's not approved by Subaru bye bye warranty. speaking from experience.
The pistons on your car are very poor as most people on here will confirm who have a 2.5s so if you try to do too much to the engine its going to suffer.