This is with Windows 10 installed to an external (USB 3) SSD with all of my bootcamp drivers up to date. The only way I was able to get consistently smooth fps without mouse movement latency problems was to play at 1280x800 with all settings set to low.
I had a blast playing this game during the stress test weekend, and for part of it I played on my rMBP 15' with the Radeon m370x. By now you must have already found articles online saying and showing you how to Play Overwatch inside a shell program called Parallels. Mac Pro.I'm not even going to list all the specs of it. Parallels Desktop 12 for Mac comes with a free one-year subscription to Acronis True Image, and those upgrading from version 10 or 11 can do so right now for £35 (around $45, AU$60). I also have 2 other HDDs each one formatted for each OS for extra space. In both cases, that also excludes the price of a Windows 7 license, which you'll need! In the end I have a drive containing a Mac OS and a windows OS during boot you can hold down option and select what drive you want, once its all setup.
Parallels, on the other hand, charges you $79.99 ($49.99 for upgrade) for its Mac virtualization product. Overwatch Specs Test For Mac Parallels Review.