They should be as such:
30-60
61-89
91-92
I've seen players outside of T8 that can do well, but they are a minority by far. The issue is AA and crit chance. They need to scale up crit chance more for the lower level players to give them a bit more impact in a match.
90's just DO NOT belong with 91-92. They don't have the 270 AA requirement on them, and most times the 90s are not even a speed bump in the match. Conversely, the gear available to someone who locked at 90 is simply unballancing to what is available to lower tier players. So, I think the best course of action is to simply have that level blacked out from BGs.