exactly for that reason, frees up hp (very little though), doesnt cavitate so you always have a nice steady flow of coolant through the motor, and yes you can run it with the engine off to help cool it down quicker which definetly helps during drag strip events or even road coarse racing.
Over all its better than the stock water pump. No downfalls at all to it other than the pumps last only soo long. The way i mounted mine was to where it was floating using the hoses to hold it up. I had no issues at all doing that. That way its not mounted to the engine and doesnt get near the vibration which would shorten the life of it. I used mine for a year and went 10k miles with zero issues. And i bought it used. Still works like a charm.