the respawn time is far too long. I don't know what system you used to count the time, but it isn't seconds. Your 3 2 1 count down lasts around 10 seconds.
You know how everytime you use one of the unlockable cars the AI gets faster cars as well? You should do that for the interceptor car too. That car just leaves all the others in the distance.
Really great game, but he expert cars need to go faster I think. Once you get used to them they are easy. Have you considered using the slingshot racing technique where they get faster as you do?