1450-1500MHz is about the average game stable. Some do better, some slightly less.
One method is to find your max Unigine Heaven/Valley core and memory clocks (core first, then memory), just before you see artifacting or crashing. Then lower core by 25 or 50MHz and memory by 100-200MHz to be assured of game stability, as gaming adds more dimensions than sequential benchmarks. You'll still have to test for yourself but that's pretty much the rule for Maxwell.
You'll want to increase the power slider above 100%. Maybe not all the way to max but a bit more. And set a custom fan profile to deal with the slightly higher temps.