Oh, I personally don't think that the matchmaking has anything to do with your SLR rank.
I was winning races just fine yesterday with barely sub five minute times on Venus at SLR rank 2. I started playing better this morning regularly winning several races and getting close to 4:30 second times on Venus... and then all of a sudden I am placing sixth with a 4:35 time on Venus.
I think the MM is independent of your SLR rank, and uses some kind of a skill based formula say using an average of your recent times combined possibly somehow with what places you have come in recently.