The only real risk I see with overdriving is that especially with an upgraded turbo (which shifts the powerband to high RPM), you could start to get cavitation in certain situations which will f*** a pump pretty good over time.
The rotating inertia of a small component like the water pump pulley is usually negligible. Underdrive pullies are mainly useful when it comes to heavily cammed engines operating well above the factory redline, where the previously mentioned problem of cavitation can occur. You may get a small amount of reduced pumping loss, but it’s not going to make a large difference, at least not on a modern engine
You don’t really get increased pumping by just spinning the water pump faster. Because of the large range of speeds, the water pump (like the oil pump) reaches max flow at less than max RPM (ideally you want max cooling before peak torque when the engine is actually stressed the hardest) and a relief valve limits the water pressure.
The only way to actually increase flow through the engine is to increase the water pressure (relief valve) forcing flow through the block, but that can do funky things to the flow path.