I think myv65 is going to jump in here, soon...
Yes, flow rate is one of the factors that I was reffering to.
The turbulence of the water increases with the flow rate, and this turbulence allows a higher heat transfer. It's not simply a matter of higher flow equals higher transfer, i.e. the proportion isn't linear.
Here's an OK article on the subject.
What I'm talking about is the efficiency of the transfer. It's not just a matter of getting a lot of heat transferred, but doing it efficiently, i.e. with minimal flow/pressure, which includes the right deltaT, and hence the right bp thickness.
Are you still up for the 3mm piece?