Question on the pipes, when these were on the car were they directly over the vent pipe on the axle, or routed around that little piece? I ask because my 3 inch OAP have a big mark and denting, about the size of the flat part of a pen, from the axle's vent pipe smacking them. Yours appear not to have that issue, so I assume they run around that part when installed?
I had these on my BMR lowered Boss for over a year with no issues. These were combo'd 13-14 GT500 Quad set up, so yes they should work fine with a stock axleback.
I used to have these on mine lowered 1" in the rear with no issues. I still have no issues with my 3" OAP. Yes they do fit any axleback that the stock ones would.
They are $339 shipped new, and I am shipping them for $240. If you want to PM me an offer, I will consider it but priced well in my opinion as this is not exactly a wearable part.