The rewards stay the same, so having a high MMR has no advantage or benefit (on the reward screen) and keeping it low is a lot more nerve sparing (e.g. playing against a low MMR antag).
Example:
MMR 35%, Gamescore 8.5 = 3500 Gold
MMR 65%, Gamescore 8.5 = 3500 Gold
It also doesn't make any difference in playing a mission with or without an Antag, because the rewards are staying the same. Makes you wonder why?
Why playing the missions on higher difficulty or under more difficult circumstances, if you are not getting rewarded for doing so.
So, the reward system really should be revised urgently!