So this is something I've been wondering for awhile but always forget to ask. Alright, replacing the downpipe (not with a shorty) takes care of both of the factory cats (other than the uppipe). An axle back take care of the muffler. So what's wrong with the stock midpipe? Is the resonator just really restrictive over an aftermarket one. Because as far as I can tell, the resonator is the only stock thing left after one replaces the dp and axleback. I'll go ahead and post the famous wrx exhaust diagram. Note: The way the diagram is labeled is somewhat misleading when it come to aftermarket parts. What they refer to as the rear exhaust pipe is usually referred to as the midpipe, because an aftermarket downpipe (not shorty) replaces both the "downpipe" and the "midpipe" in the diagram.