I just started OC'ing with the AMD RX480 8GB. Neafty card, however, the stock cooler is unable to keep the damn thing at stock clocks, i.e it will throttle and run basicly on lower specs. It's hard to actually hold 1350MHz.

Try a simple furmark session, you can see the core clock going up and down, even with power target +50%.

Hwinfo64 and with sensor readment, will show some interesting results as well. If your oc'ing the memory, you'll note the GPU memory errors. If you get lots of errors it will actually hurt performance for 'retrying' that same failed bit over and over again.