I've got up to 1850 in TS/TA alone last season. Like I said, it's not about carrying a team, but getting lucky. You get good teammates, avoid facing teams, get quitters on the other side, etc.
Which can just as quickly happen to you a few games in a row and you bottom out, lol. I was cruising to 1900 at one point last season and got back to back multiple quitters early in matches. Solo MM is a very streaky experience 99% of the time, heh.
I'd have to assume there are very few people who could legitimately solo their way to Champion barring early placement cheese, lol.