Does impeller torque stay constant as RPMs rise after cavitation or also increase?
I have an aftermarket pump pully i'd like to use on a 360LA but it is a bit smaller diameter than stock.

You guys that weld a disk to the blades to stop cavitation, why and what problem did it solve?
Some water pumps have more blades than others-is this for low performance engines only?
Could water pressure blow hoses off if you overdrive the pump and don't allow cavitation?

At what RPM does a typical parts store reman water pump start to rob HP needlessly?

cavitation is bad.
it is actually boiling of the water due to low pressure on the following side of the blade. and the subsequent crushing of that bubble from steam back into water from the blade moving towards it causing a high pressure front.
This can damage the blade face eventually eroding the blade face.

the plate is supposed to help eliminate this by limiting the amount of pressure drop as the blade moves away from an area. Because the volume is more fixed in that space, it cannot drop as far as if it were wide open.

If I were to guess, you would use less power moving due to cavitation rather than more. Since vapor is compressible it would not require much effort to move into that space.

the 8 blade wp, spun slower, for non-a/c cars.

6 vane spun faster for a/c cars.

with much of above. But OP should also not overlook, the higher the operating static pressure of the system, the more difficult it is for cavitation to begin.

I’m running a small block 68 A/C water pump pulley which is small. I shift it at 6400 and cross the stripe at 6500-6600. I pulled the pump and I had cavitation pits in the front cover. Enough that I’m concerned it could eat through the cover. Installed a anti cavitation plate and went to smaller crank pulley and the pits have not progressed.

Exactly. Cavitation occurred because of the piss poor design of the Chrysler water pump. Get a quality high flow water pump and spin it up. It won’t cavitate.