I dislike rubberbanding in the custom cars grand prix mode because upgrading your top speed feels worthless. but i guess it would be too detrimental to slower cars in other modes

It could be improved to make catching up with the AI drivers easier if messing up. One of the things I like about the rubberbanding is that it no longer makes you outrun the competition by hundreds of meters but get to fight with them for the best position, but that comes at the expense of making them pretty much impossible to catch up with.

An additional difficulty setting specifically for rubberbanding (Off, Benefit Player, Neutral, Benefit AI) could solve the problem and combined with the existing speed classes let the game be fully tuneable for every player with respect to skill level, play style, and desired experience.

So all hope is not lost.