I got a bunch of PowerColor R9 290 cards, and noticed they run best at a engine/mem ratio of ~0.74. You could probably get higher than 800 if you kept that ratio, but overclocked a little bit.

All my settings run at Powertune @ +20
1105/1500 gets me 855
1075/1450 gets me 835
1050/1400 gets me 810
1025/1350 gets me 795

I would love to run them at 850+ each, but the temps are too high for my liking right now, so I have to downclock. We're going to move them in a couple of weeks and install some more AC units. Hopefully between that and 2 weeks of burn in will allow us to overclock even further.