Isn't that body a mod? Because if it is, then is not the job of the devs to fix it. Otherwise, could you send me the car exported and I can check the Lua file, maybe there is a simple error in the Cd or lift calculations that you can fix.
About the other things, those are suggestions and probably are better suited to the suggestions subforum. But anyways, about the transmission, change in peak power isn't always the best idea, let's say you peak power at 6,000 rpm and the rpm limit is at 6,500 rpm, if you change from 2nd to 3rd at 6,000 your rpm will drop to about 4000rpm, 2000 rpm below the peak power, but if you change at 6,500rpm then you rpm drop to 4,500 only 1,500 rpm below (meaning it'll get faster to peak power) and you'll be in a range with more power. Secondly, we'll, you just can modify the Rev limit to set where do you want to change gears, if you still want to change at peak power, well, just cut the rpms at peak power and there it is.
And about making power at 9000rpm but change gears way above that, must likely during shifting, your rpms will drop above peak power, where the engine isnt quite efficient (less power more consumption).
In game, in the engine designer the performance index tell you when you set the Rev limit to get best performance.