dude overclocking ure core in respective with shader clock
this are the steps or core clock .... 648, 675, 680, 702 etc
this would give significant advantage
being inbetween them has no advantage
same goes with ure clocks for mem.
advice you to use atitools for scanning of artifacts
and rivatuner 2.04 which unlocked the shader clock... u got me there..
and everest to monitor shader clock and mem clocks...
also for overclocking try to run in sync with ure rams with the lowest timing..