Would you mind posting your entire set up?
Not at all, I've two main boards Z87 Pro, each have 2 R9 280x cards. I'm waiting for powered risers to add more cards. I've a 1200 watt PSU on each, so I'm not expecting problems upto 4 cards. The board has 6 PCIe slots.
Currently I'm getting 750-760 khs out each of these cards with the following settings in guiminder-scrypt using CGMiner
--gpu-platform 1 -d 0 -w 256 -v 1 -I 13 -g 2 -l 1 -T --lookup-gap 2 --gpu-powertune -20,-20 --auto-fan --temp-target 65,65 --temp-overheat 85,85 --temp-cutoff 90,90 --gpu-memclock 1500,1500 --gpu-engine 1080,1080 --expiry 1 --scan-time 1 --queue 0 --no-submit-stale --thread-concurrency 11200