I didn't know they made overdrive water pump pulleys. Underdrive yes. Quick search didn't show anything talking about optimum RPM for a SBC pump, although someone like Stewart or Flowkooler could probably provide some details.
At some point I imagine the setup will simply be unable to move more coolant due to the restriction at the thermostat and the radiator outlet. The advantage to an overdrive pulley would be at low engine RPM's, but that will come with some efficiency (HP) loss.
Unlikely it would run too cool, the thermostat controls the minimum temp of the engine.
There are a host of variables that need identified and considered before an educated guess could be taken. Somewhere around 76-100GPM seems to be a decent figure given the inlet/outlet diameters of the radiator, water pump, and/or thermostat housing, but the thermostat itself has to be a significant flow restrictor.
A high flow pump should increase cooling efficiency at low RPM by itself. While you may still lose some HP from it's use, it would seem unwise to modify both items at once. I'd do the water pump first and see how it works out. At least then you'd have an idea how much of a difference (if any) the pump made, and then could potentially do the same for the pulley, if you decided to. It's going to be pretty hard to determine without some pretty serious test equipment. You'd have to keep the same starting engine temp, same engine load (same stretch of road, same RPM) and same ambient temp at a minimum, and then somehow figure out how to accurately measure coolant temp over time.
Don't get me wrong, I love this sort of stuff, I just don't have time to dig into all of it. EFI makes it a bit easier, because you can track the coolant temp over time and against load if you have the datalogging tools.
