water pump

I have done a lot of experimenting with w/pumps over the years, trying to cure hot running engines. I found the biggest problem was the gap between the back [ or front, depending on car/engine model ] of the moving impeller blades & the housing. The bigger that gap is, the greater the amount of water that is NOT captured by the scooping action of the blades........& just gets churned up but does NOT get pumped through the engine which is supposed to be the goal. I do not believe the number or shape of the blades makes much difference......if the water has an escape route. I see a lot of talk about cavitation. How would the average joe know if the pump is cavitating?
I make or buy a plate to close the above mentioned gap. I braze it in place usually, but sometimes because of the pump design I screw it in place. One engine I modified this way dropped the temp by over 20; most, not by that much, but always some temp reduction.