I have a similar problem with dual HIS 280x (i have a total of 4 actually). On two different boards with two different power supplies ( a Thermaltake 750 and a Corsaire 1000). A single card runs at 730 and cool, dual cards drop to 600/500 or 730/630 with some extensive cgminer tweaking and very hot. I don't know if it's a PSU issue, doesn't look like it in my case.