I have 6 4 on the cool side one of which is very cool and 2 on the warm side though not over about 25 percent.
All 6 do fine at this point but then they are all level 23 or above now.
If you feel that one of your drivers is no good then retire him and get another. Enter him in the highest level race you can where you can also enter a dominate car for him to drive and just tell him to pace down on the first race. He will level up in no time.
My 6th driver is right up there with the rest of them now even though he has less than 20 races and some of the others have closer to 100.