I run 3x7950, one 280x, and one 7870 on my z77 gigabyte board.
The key for me was to use a separate instance of cgminer for the 280x. My understanding is that each instance only supports one thread setting, so the 7950s and the 7870 run at -g 1, the 280x at -g 2.
I haven't noticed any effects on hash rate, everything is performing up to spec.
Noted - very useful info, thanks!