Not a stock cooler, ok that narrows it down.
50c is pretty low, so you have plenty of headroom there. Keep going and testing "Properly" until you either start hitting high temps (85-90c or whatever you are comfortable with), or until the CPU won't OC any further.
I can't really recommend a max voltage as you still haven't answered what cooler you are running.
"FPS goes from 300 to 200 or less in CS:GO" - Why do you need 300fps, do you have a 300HZ monitor/panel???