The specific racing class I plan to do only allows a drop in air filter and exhaust from the turbo back as long as there is a cat. That being said, I have a litespec cat back and am planning on a milspec HFC.

My question is with the HFC and just a drop in filter, would it be worthwhile to get a down pipe/O2 housing or not? I like the idea of being able to easily swap the stock cat for the HFC and flash a base map if I need warranty work done. I would however go with the down pipe/O2 housing if there was a lot to gain. Ideally I just want to put the meager bolt ons allowed for this class, and get it tuned once and be done with it.

I'm willing to give up a few horse power to be able to easily go back to near stock in case I need to use the warranty. What kind of gains will the down pipe/O2 housing give me with it being chocked on both end by the stock air box and a cat?