are you talking about alphacools new VPP755 model?? keep an eye on them if you are. I had 1 of each version fail on me rather quickly. The rev 1 the impeller kept popping off the retaining clip. The rev 2 would sometimes not turn on with the system.
I wouldnt worry about trying to control the pump via pwm off the gpu, for one it will look like crap with a wire hanging off the gpu like that, secondly the temps you'll get just running water on a gpu will not change much if any at all by running the pump faster. For most we find a set and forget speed to be equally superior to having the pump ramp up and down. All you get is more pump noise without any difference in temps.