I had to underclock my cores in both my GPU's as they sit 100% next to each-other. (i have a order of riser cables, though probably 2 weeks till i receive them)
With that said when i underclocked BFGMiner gave me much better hashing than what cgminer did with the same configuration, running: 2x - Asus Matrix Radeon R9 280x
Am concern with the need to tweak the cores. I read that phoenix has these settings below, however am not sure if the processing is more efficient than the other two.
Vectors on / Aggression Default / FastLoop off = ~29.5MH/s
Vectors off / Aggression Default / FastLoop off = ~32MH/s
Vectors off / Aggression Default / FastLoop on = ~41MH/s
Vectors off / Aggression 3 / FastLoop on = ~46MH/s desktop lag barely noticeable
Vectors off / Aggression 4 / FastLoop on = ~47.5MH/s with noticeable desktop lag
Vectors off / Aggression 5 / FastLoop on = ~49.5MH/s but quite a bit of desktop lag
Vectors off / Aggression 7 / FastLoop off = ~50MH/s but extreme desktop lag
Vectors off / Aggression 7 / FastLoop on = ~51MH/s but extreme desktop lag