Here’s some data from my support games in the past two seasons.
spreadsheets/d/1pQ-MU-mQcgOJ9kzj_DPD-HfI23KUbBDo1rkISRE6VXQ/edit?usp=sharing
(Not allowed to post links, copy/paste to docs google com)
This account was played exclusively 3-4 stacking with the same friends all the time. They are lower ranked than I am, so I end up being above lobby average by a couple hundred SR almost every game. Despite going 68-57-3 my rating has actually gone down slightly. We win 20% more games than we lose, but losing costs me 20% more SR than winning gets me. My friends climbed about 150SR in these games, so the gap is trending downwards.
For comparison, I have another account that I play solo queue only. Same heroes, similar stats. Lobby average is never more than 40SR away from mine. It has a 53% win rate and wins give me about 4SR more than losses cost me, on average, which is the opposite of the group account. I consistently rank about a tier higher on the solo queue account.