Faster with wrong impeller does not mean faster water if cavitation occurs. Hard to really know what they were doing unless an engineer has the notes. Proper cooling, not too much power loss, versus AC with more power loss and heat generation at higher idle speed. I guess it all depends what you want to do with the car, but once you have sufficient cooling rate, getting more is not a benefit. Is car going to idle a lot in city driving, or cruising at higher rpm, or spend most time at high rpm doing the 1/4 mile.