The boost meter is very well implemented!
I did not find a level selection but I guess this is because of the demo?

You got my vote on Greenlight but sadly I have to say I don't enjoy the game that much anymore, and that's simply because of the new Kar handling, it's way too direct for my taste. I remember playing an early version of your game with a friend of mine some years ago in front of my PC when we should be doing stuff for university, and the fun and especially the tension arose just because of the drifting and general handling of the cars.
What I would suggest for you is to keep your movement right now for concrete/ asphalt grounds and implement the GS handling for your default mud/ ground levels. Simplicity in terms of direct handling is IMO not a good thing, that's like making the movement in a jump'n'run/ side-scroller less interesting on purpose.

Anyway, I wish you good luck with KarBOOM!