In order to stream Overwatch or any game with high camera motion at 720p 60fps you need 4000+kbps bitrate.
As a non partner you aren't supposed to go higher then 3500kbps and, at this bitrate, 720p 45fps look better then 60fps.
If you dont have transcoding yet dont go higher then 2500kbps. And stay at 720p 30fps.
I dont think you will be able to reach "Medium" if you play and stream on the same PC while not restraining OBS CPU core usage.