Are you wanting to control the D5 pump? The AC flavor of the D5 uses a speed adjustment knob/screw on the back of the pump to change the speed. It is not a PWM controled pump.
I just tried it using a Poweradjust and was quite dissapointed with the results. It seems the adjustment on the back of the pump does more than control the voltage because the RPM range is 1800 - 4800 RPM. If you lower the voltage below 7 volts (3300 RPM or about equivalant to setting 3 on the pump) the pump will turn off.
Changing the flow rate won't help with temps. I run my pump at about 2.5 or 2900 RPM which on my loop yields 1.90 l/m or 0.5 gal/min. It doesn't sound like a lot but I've run the pump from setting 1 to 5 and it hasn't changed my temps in the slightest.
Buy a D5, set the speed to something that works and forget it.