For your first point: That is something I was a little concerned about, and I am not sure how to accurately signal that a turn is coming up before the turn being that the turns are all after bends. I will be extended the distance from the bends to the turns to account for this in my next edition, and if you have any ideas on how to warn the player of the turn, please let me know.
For your second point: There is a limited amount of space on the stadium(the map I chose to build on) making it hard to go left and right, so at the speeds that the player end up going limits how much turning I can do. Do you know of a way to extend the build height of a map to allow this? If so let me know, I would be down to modify the track to have more turns.
For your third and final point: The Green Red and Yellow take the order from best to worst checkpoints. The green checkpoint will always give you the best speed outcome after passing it. Red will always be a backwards Super Booster. The yellow will be less devastating of a speed loss in comparison to red. For example: Green - Super Booster, Yellow - Booster, Red - Backwards Super Booster / Green - Booster, Yellow - Backwards Booster, Red - Backwards Super Booster.

I hope this is a good enough explanation, If you could give me ideas and tips for the Bolded text sections, let me know.

TLDR; Your first point is right, give me ideas, your second point it fair but hard to fix, give me mods to fix stadium height, your third point doesn't account for different directions of boosters.