The XR2 has a 17mm rear stabilizer bar, while the MTX N/A has a 14mm, and the ATX N/A has no stabilizer bar at all.
But for some reason my car came with a 12mm, which is really weird and I don't think it's a stock one. I've been looking at summitracing website and they list a 19mm, 24mm, and a 25mm one as possible upgrades for the rear.