Only the muffler, as far as I know, is stainless. The rest of the exhaust is regular old steel. Here's a diagram of the exhaust system:
http://www.subaruparts.com/diag/?mod...category=440-A
The center pipe, after the cat and before the muffler, is all one piece - including the resonator. But if the front part of the pipe is all that's bad, an exhaust shop should still be able to cut out the bad parts and weld in some new pipe. If the resonator itself is bad, you can have that chopped out and replaced with just a straight piece of pipe. Owners who have done this report little, if any, increase in exhaust noise.
By the way, we have a tech forum for these questions. Please direct any similar questions there in the future.