So at the same flow rate, with the same amount of heat, the head pressure will make the Mag 3 better? I've never seen any testing done on this, and I've always sort've assumed that the only thing that head pressure does is allow you a higher flow. I think you have to go into the high restriction zone to find a major difference, but most people stick with medium to low resistance blocks. A WhiteWater has 1.5 ftH2O head loss at 1GPM, and a Silverprop GPU block has approximately 18" drop also (Actually, this is based upon their CPU block, but they're designed the same) and someone would be a fool to get a NB block with a pressure drop greater than 10 or 12" at 1GPM. That's 4 feet drop, and let's add on 6" for the tubing. So, we have 4.5' drop, at 1GPM, and the Eheim 1250 delivers that easily, and the Mag 3 gets that easily as well. Neither of them will make it to 2GPM. Now, based upon testing done by BillA, the difference between 1GPM and 2GPM is very small, and with even less of a difference between the flow rates, the temperature difference is small, possibly even negligible... *Is surprised at yet another realization*
Edit: Ugh... I'm not thinking right today, but I think it's still fairly accurate.