I find that the experience I receive is being wasted. I would like to see an update in the future that either doesn't limit the amount of experience you can acquire, convert experience into currency, or allow the transfer of experience to friends.
Transfer to friends would be a bad idea. Not really giving them legit levels then.
However i agree with you on the convert to currency thing. This would help the level 40's that have had enough of grinding to lower the amount needed for cash, keeping them interested in the game enough to get the car collection.
A better system would be to have many factors (such as power of the car related to the opposition, how far ahead you are etc) to give you an average 'ranking' (for instance, out of 100. If you use the same cars as the A.I, but you put in a massive lead, you'd be closer to 100, but if you had a car ridiculously more powerful and put in the same lead, you'd be closer to 0). With each race your 'ranking' increases or decreases based on these factors (starting at 50, for instance).