I tested this out after seeing some murmurs on the forums about this yesterday.
Final results: When a bar has been "maxed" out, it can still be upgraded, but it won't be visually shown at Los Santos.
If this has been done before, I haven't really seen it around. I know as of yesterday there was a discussion, so I decided to go ahead and do this.
I have more testing videos coming soon regarding exact testing/comparisons of simultaneous accelerations/speeds/turns of all the super/hyper cars at once over the next couple days.
The 9F is still my favorite car. Cheetah is, IMO, the "best" car and combination of spd/acce/handling/braking. Yours?