Most catback exhaust systems don't void your warranty, it kind of depends on the dealer and if it's installed by them or not.

 

If you do a search you will find a ton of posts relating to warranty coverage. What it comes down to is that a catback will not legally void the warranty. However, you will encounter dealers, as you already have, who will try to deny you warranty coverage based on even the smallest, most inconsequential modifications. If you are concerned about "voiding" your warranty, I suggest you consult with the dealer who does the work on your car before making the modification. If they say it voids the warranty, then you're going to have a hell of a time convincing them to do warranty work later. I personally have pretty much given up on Subaru dealers and decided that I don't really need the warranty.