But how do you quantify playing "great"? Last night on Eichenwalde I had a game where I went 9-1 across both attack and defend as Zenyatta and only got one medal for 6,000 healing. However I was discording like crazy, kept our Widowmaker and Reinhardt alive constantly so we could hold the first choke point, kept an enemy Reaper at bay by discording him and making him run away every time he tried to flank, and had a key Transcendence where I protected our Reinhardt so he could kill the Bastion that was the centerpiece of the enemy's defense, which led to our win. Barely any of that shows up as quantifiable stats, though - so would I gain or lose SR in that scenario? It was a clear victory on my team's part and if I hadn't performed how I did as the team's only healer, they probably would have at least lost the first capture point.
I can see still getting positive SR if you go like 50-5 on KOTH but still lose, but the team's win/loss should definitely still count towards what you get as well.