What asic is your card?
I highly recommend using OCCT GPU test to check for core stability in Hawaii. Its very quick and easy to check gpu stability. It picks up tiny little graphical errors that would otherwise be hard to notice while testing for stability while gaming. Any errors detected, you need more core voltage or a lower core clock. Its useless for memory though, so stick with core.
I can do 1025/1250 @ Core voltage -0.050mv and AUX voltage at -0.087mv.
For 1100/1250 i need voltage (Core and Aux) at stock levels. This has been tested using OCCT's error checking. If you can pass 10-15 minutes at the setting i posted above you're rock solid stable. You could actually maybe lower voltage a tad further and be stable, but eventually you may get artifacts. Whether you actually notice these while gaming depends on the level of stability. If you get just a couple of errors you probably won't notice. If you get thousands of errors you likely will notice.