Only thing you can upgrade on that RE-Rx7 is : Rims, turbo to stage 3, gear ratio, tyres, and muffler. That's about it might have forgot something but dont think so, can't repaint, can't change aero or anything. Can't make it lighter neither which sucks because it's obviously the street legal car (I'm thinking the one Suenaga used while drifting in street legal in 2008) and not the touge monster we all used to see. If you look at interior there's still a lot of stuff in it that could be taken off to make it lighter.
I might do what you said actually with index. I'll do my next update like that will be more bandwith friendly that way too I guess.