Running 1 gtx 590 and 2 gtx 470's in PPRO....
see my above post for speedgrade issues for multiple GPU's...it likes the monitors involved to be on the same card
not sure how much it helps, speedgrade may only use 1 gpu to accelerate (i suspect as it had a lot of previous issues with multi GPU's)....I would think PPRO uses all devices to accelerate, but have not disabled cards to test performance differences.
1 thing is for sure....with RED footage (which is debayered using red sdk) all GPU's are used in PPRO as ove CC 2014
Not sure if speedgrade is using the newest SDK with the GPU acceleration, but PPRO CC 2014 does for sure.