Only problem I see is with slow 4-door cars in IV. 50 % of the players will get killed in first 3 CP's after start, after that it's just one leader car with 4 shooters and 30 cars with single players and ramfest.
What comes to my mind is some sort of force field, which can be activated like 2 times in one race for like 15 seconds or so. The force field could protect the car/players. So you can decide when you need it, for example if the driver fucks up and the pack catches up. But the amount of work which would have to be put into this would be too much in comparison to the outcome.
I see no way to even out the difference between godlike deagle shooters and noob uzi shooters.