But each gives 720 kh / s to 730 kh / s
Maximum temperature is 78 degrees.
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 !
750/760 on the first OC run while I was setting them up for someone else. Due to lack of cooling that the person provided I did not push them further.