Given that the most recent Forza Friday stream mentioned that matchmaking currently prioritises lobby size over everything else, I would say that Safety Rating’s impact on who you race with/against isn’t as significant as it’s expected to be.
I’m S/4995 at the moment and half of each race I do is my friend group (all A/S drivers), so it’s been a while since I’ve come across significantly dirty drivers, but that’s a sample size of one.