When I tried to run the ATI 5450 cards in GPU with the SMP, it reduced SMP performance by 40%. To put that in perspective, a 24 hour job become a 34 hour job. Since the bonus is calculated by how fast you return the WU, it actually makes less when running both than running just the SMP.
I see, must be an ATI thing. I paused the SMP job for the whole GPU WU and didn't see any difference in total time TPF. Must be the GTX4## advantage.