Similar issue, let me try to revive this thread
Powercolor 280x, 13.11, win7 ultimate 64bit, cgminer 3.72
In all my multi gpu rigs, one card is always couple of percent slower, i.e. all cards are 720khash/s, except of one which is 670-90khash/s
one cgminer instance per gpu. Exactly the same cgminer.conf with all gpus, and all gpu’s are identical.
edit: will start another thread… no need to reply here…