SR is a tricky subject especially now penalties have harsh SR deductions while still being assigned wrong far more often than not.
SR is the first factor in matchmaking and is the reason why the competition often seems completely unfair.
Any SR Down (penalty or not), getting the orange SR Down arrow for contact with another car deducts 10 SR.
Race A and B usually only give out 4 to 6 SR for a clean race, Race C nowadays is worth around 12 points per race.
This combined with penalties being assigned wrongly most of the time, does not guarantee any better racing at SR.S than at SR.C :/
In my experience, racing as DR.B SR.C is much better nowadays than at DR.A SR.S, sad but it's been like that for a couple months now.
Time of day, whether you do Race A/B or C make a big difference in where your SR goes, it has little left to do with sportmanship or driving by generally accepted racing rules.
https://f1metrics.wordpress.com/2014/08/28/the-rules-of-racing/
Matchmaking works by collecting all entries between 90 and 99 SR, sort on DR, create rooms.
Then collect all entries between 80 and 89 SR, sort on DR, add the highest DR to the left over spots in the last room created and continue.
This results in for example getting rooms like
1. A+/S90+ to B/S90+
2. B/S90+ to D/S90+
3. A+/S80+ with D/S90+ (the last D/S90+ room had spots left over which get filled with the highest DR in 80-89 range creating a A+ with D room)
4. A/S80+ to C/S80+
5. A/S70+ with C/S80+ to D/S80+ (the last 80-89 SR room had spots left over which get filled by the highest DR in 70-79 range)
etc
Matchmaking only really works in the 90 to 99 SR room, the rest all easily get messed up due to how the server fills up rooms.
Still when there aren't that many entries the top room is usually A+/S90+ to D/S90+, you don't stand a chance.
SR.B is the largest group, which makes the competition the closest. Most people to choose from to make evenly matched rooms. That is, if it weren't for DR resets and alt accounts coming through. (people that are labeled DR.B but drive like DR.A or A+)
Closer matched in pace also means more chance at contact and bad penalties, hence SR.B is such a difficult place to advance from.
Head spinning yet?
Don't worry too much about SR, D/S is the most demotivating place to be, usually getting matched with rabbits that are impossible to catch leading to getting timed out before the finish. D/S is only good for maintaining SR.99 since the field spreads out so quickly reducing chance of contact.