Here is what I've done to achieve max hashrate for this GPU. I set the core voltage to 1250, and the core clock to the max of 1200Mhz. Works without crashing. Of course you'll wanna adjust the fan speed too; I personally don't mind the noise, so I have it set to 100% speed when it hits 60C.

This setting gets me 612 MH/s, with the flag "-w 64." And the temperature is the same for me, 76 - 78C while mining for a while.

So if you wanna get another 100 MH/s, this is what ya do. Cheers!

*NOOB WARNING: Do not attempt to overclock your graphics card if you don't know what your doing! It voids any warranty you have on it, and can break the card if you do it wrong.*