There is a document that discusses most of what we know about the MMR/SR system. Including this point:
Also that performance modifier is based on,
… not on medals. Medals have nothing to do with it. On-fire percentage is roughly but not perfectly correlated with performance modifiers.
Personally, I think at low ranks it’s very difficult to earn a good performance modifier without carrying your team pretty hard, and on a single hero pretty difficult to earn a noticeably negative performance modifier without throwing pretty hard.
If there’s a problem with performance modifier calculations for players who switch heroes, that should definitely be addressed.
Given the number of deliberate deranking throwers in low ranks, and dominating alt-accounts going unranked-to-M/GM in middle ranks, I’m in favor of keeping PBSR. It gives players a way to affect their own SR even when these unfair factors drag their win rate towards 50%*.
*Towards 50% because if there is a thrower or a smurf in your game, it’s nearly a 50% chance whether they force the game towards a win or a loss for you depending on which you have and which side they are on.