I think the system should also be slightly extended, you cant put all drivers in just 6 categories. There is too much variety, and especialy on higher skill levels its down into the details. I would suggest adding 2 new categories: F and S+
To me i would put the following ranking system (and with that, double the scale to go between 0-200 instead of 0-100)
F is specificly to catch the worst possible drivers, and is only ment to be quickly get out of, but also to protect.
0-20 F: All collisions disabled
21-60 E: Most collisions disabled, collisions are only in effect when someone is slightly bumping into someone else, and only affect the offender (kinda how in old gran turismo's the cars acted like solid bricks and could be used as steer assists). Collisions do still affect the SR on that (so they cant rise to D by using this method).
61-100 D: Collisions at low speed diffirences now affect both. If someone is forced off the track by a bump, the one forcing him off takes double the SR penalty
101-130 C: Collisions at high speed diffirences cause the higher speed player to become a ghost, all other collisions are in effect for both.
131-160 B: All collisions are enabled, taking someone out as a missile causes a drop of (at least 15 SR - 2 of these hits and you are back into C), the one hit by a missile is guarded against the SR loss. Forcing someone off the track causes a drop of at least 5 SR (depends on the duration that the other player is off the track, maxed at 15 per hit which is similar to missiles).
161-185 A: All collisions are equal to B, being a missile once takes you back into B. Slight collisions should still be managable, but any heavy hit will hinder a lot, being hit by a missile will also affect your rating here (at a reduced value).
186-195 S: Similar to A, in the last lap penalties are doubled.
196-200: S+: Similar to S, but nearly every mistake takes you back to S.
The reason to go to the 200 scale is to be able to finetune the S vs S+ ranking (together they are only 15 points in value, which together is equal to A), which is only a system that should keep the S/S+ racers to remain motivated to keep the clean driving as there is simply no gracing point for the rating in S+, except for some minor bumps.
The reason to keep F low is because its still equal to E in its effect to other players. But F forces players to learn to follow the track since they can not use other players as assistant. And at E learns players to avoid collisions even if it doesnt affect other players (and if they dont want to, they at least wont be hurting the others)