We might be confused here. My gen 5 has a three piece. The headers, which look to have a cat welded to them under the engine, a mid pipe with a muffler that might have additional cats in it, and the end can (I call that a slip on) which is just a straight through tube for some additional noise abatement. If you get a straight through mid pipe to go with the slip on, what I called a 3/4 system, though really more like 2/3, then you'll have something to work with, not the least of which should be a properly sized O2 bung for fitting the woolich auto tune kit. If you don't want to go for the auto tune hardware, at least 2 maps can be had on the woolich mapshare page for a graves "3/4" system. Being a natural skeptic, I would want to ask Woolich their opinion of the effect that forward cat has on the accuracy of AFR readings.