My thought is keep the fans attached, but let the motherboard throttle the fans like it would with a normal cooler using the load ramp, not temps. To make sure it's more agressively marked in Windows 7, simply make a custom power Option and under CPU cooling set it for ACTIVE. Gonna try this with my outter fan probably tonight or tomorrow, since the outside temps are going to be cooler. This way Ambient temps wont make the Radiator temps shoot up while testing.
I live in Norway, so i just open the window and it takes 15 min and my room temp is around 0 degrees :-)
I tried what you sayed, but the problem whit the 620 is that you can only control the pump. And when going under 1000rpm its start making gurgling sound if you understand what i mean. tick tick tick tick... And from 1000-1400rpm, the fan doesnt change that mutch, pritty mutch going on full speed no matter what it seems.
So i think i will remove the stock fan, or perhaps put it in front of the case, and use 2 diffrent fans whit 3 speed lvls. And have the pump connected to the mb only, going on 1400rpm.