Yes, being able to transfer excess CarXP would be great.
Another potential avenue to explore would not be to buy Car XP with real money, but instead it buy with in-game CR. If it cost me 20,000 CR to bring a car up 1 level (a million to go all the way to 50) I’d do it.
There’s lots of things that could be done to speed up the XP earning process, and another thing I think would go down well is if every level gave a flat 400 CP.
At the moment you need to get to Level 46 to get 10,000 CP (which is roughly enough to do some significant conversion builds). Awarding 400 per level would put that down to Level 25, which is about an hour and a bit of driving, and gives you enough points to experiment with conversions the moment you unlock them.
The entire system isn’t bad in theory, but it is in execution, simply because everything takes so long to do. When people are encouraged to automate a levelling system, it’s not a good enough levelling system to engage with organically.