Interesting... does the 1250 increase flow compared to the 1048? Depends on the flow restriction, as was said. Else all that power is just converted into useless heat.
But aside from that, who says that increased flow means increased cooling?
Let's find out in a thought experiment. Let's say we bolt on a super-duper pump which makes the coolant flow at infinite speed (beyond speed of light, say, Ludicrous Speed). For sakes of argument we will assume this is not going to cause problems with friction, pressure, heat, time-paradoxes etc. blasting your rig into far orbit, because we slapped a SEP (Somebody Else's Problem) field generator on it.
What you will get is that heat will still transfer from block to coolant (as it should) and will transfer from coolant to rad to air (as it also should). The coolant will achieve a certain even temperature overall.
Now we slow down the flow a lot, down to, say, an Eheim 1048. We find that the coolant in the block will get a bit warmer because it hangs around there longer, but that's OK: it will lose that extra heat in the rad by virtue of the fact that it hangs around there longer too (in fact, the hotter the coolant, the greater the transfer of heat). Provided that it doesn't stand still, the slower the flow, the bigger the coolant temperature difference between block and rad, but the overall cooling capacity is still the same. Q.E.D.
