I'd like to add three things:
1) Subaru cannot actually "void" you warranty for aftermarket parts. However, they can and will refuse to make repairs under warranty for problems associated with aftermarket enhancements. But this same dealership will most likely repair your broken power window and alarm under warranty despite your catless up pipe. (Like my service manager just did, despite installing the up pipe himself.)
2) When someone says "the M-M Act doesn't allow Subaru to void the warranty for aftermarket parts," all that really means is a court will not legally enforce Subaru's decision. That is, Subaru can declare that your warranty is voided. You can scream about the M-M Act till you're blue in the face. Subaru will again say that your warranty is voided. At this point you can either pay for the repair or get a lawyer involved. When anyone says they can or can't do something legally all it really means to a corporation or lawyer is what a court can or can't enforce. That is, when a court actually gets involved. Till then you can do as you like.
3) Finally, most of the answers here will tell you to look for a mod-friendly dealership. There are more and more dealerships that offer aftermarket installations. Mine does, for example. And they appear to be the most reasonable.