I think armor won't solve the problem.
Deagle is kinda overpowered for the current race mode (mass starts, slow cars, long straights, etc.). Two shots without armor (or 4 with) and the driver is dead. Most IV races now finish after 20 seconds and the rest ist just the winner car driving to finish line with the rest of the players restart as drivers and following a minute behind.
What about some health bonus system?
The shooters get back a little health (like + 10 %) for a kill, and the driver (as the most endangered player) gets around + 25 % health with every checkpoint he reaches.
This way, the drivers are better protected and would most likely survive the race. Don't forget, it's not just the shooting the driver has to dodge, but also the ramming.