Unfortunately, I feel your pain when it comes to spending $$ on the Hobby Design SLR 722GT kit and being SEVERELY disappointed. I did the same thing you had done for the rear bumper. I cut the plastic Tamiya one off, then realized I was better off when comparing the resin one on. I gave up on it awhile back and ended up selling it.
I'd love to build a 722GT, but next time it won't be using the Hobby Design transkit. I think I'd be better off modifying it myself.
GOOD LUCK! It sounds like a very unique project.