I don't think so. I'm SR S and the others DR B players in mixed lobbies were also SR S, but SR of DR A players was varying from B to S and A+'s SR between A and S.
I start to think that it may be due to two factors: my QT was yesterday within the 500 fastest overall (within 150 fastest B) coupled with the fact that I'm near to rank up to DR A. I've seen there's always ar 1 sec gap between poleman and 10th-12th at start. That is also a tool PD uses in matchmaking, I guess (not so sure btw). I fear that if I go below 56 with my QT I'll end up in lobbies with majority of A+ drivers. I'm now really thinking to "sandbag" with my improvements even if I know where to gain that 1/2 sec left.