On our Sapphire R9 280X the maximum we would squeeze was around 730khs so I'd say your perf are in the average.
Some manage more, but it really depends of the type of RAM you get (not all cards are born equal).

I'd recommend to put 140mm fans in front of exhaust of the card, to extract the heat. It will lower the temp to 72°C.
We also have fans on top of cards blowing in the space between them, help also.

Best of luck !