I never heard mower blade cavitation suggested from any source, being driven by more power than necessary or anything else. I am an engineer but NOT one with fluid dynamics credentials. In my opinion mower blade "cavitation" is very far-fetched and cannot happen. Cavitation involves bubble formation due to extreme sudden negative pressures in a liquid. When the bubbles collapse (essentially out of control) it can be violent enough to cause erosion of the container material or vessel involved. That is common with things like boat engine props, turbines, etc. Mower blades running in wet grass are going to slow down regardless of other factors. There is nothing in the vicinity to create THAT much pressure. High HP input doesn't really matter because that high HP is not going to create high enough rpm and blade tip speed to even think about some effect like cavitation. The high HP would certainly drive the blades through heavier load (like deep wet grass, etc.) but that isn't going to produce high pressure bubbles collapsing hard enough to eat away metal... in my opinion. A fluid dynamics expert in some pump design group could put this to rest once and for all but I say it is very far fetched.
