[Butcher]

how mugh of a degradation, you're likely to get some from a larger pump putting out more heat (this may be more significant than it seems at first glance), especially if it was a submerged pump.
I've seen data that used the same pump with a restriction in the pipe to simulate lower flows, they showed no degradation at higher rates, but above a certain point no real improvement - much as you would expect.
It goes both ways, less heat dropped in the rad at higher flow, but also less heat picked up in the block, so the net temp shouldn't be higher, lower if anything as the water is in the block less time so can't get so warm.
Additionally, if more flow is bad why do big fans work? Surely if you use a too big fan the air doesn't have enough time to pick up heat from the heatsink by your theory, it just doesn't add up when you think about it.