Jugger, you've made me realize that I know little or nothing about high-flow water pumps even after being in this hobby for over 50 years.
I will share with you what makes sense to me. Filling the pump cavity with as much impeller as is possible makes sense. Making sure the impeller is a tight fit on the driveshaft makes sense. Using a solid sheet metal backing plate on the impeller to prevent water from circulating inside the pump cavity makes sense. Possibly increasing the drive ratio off the crank makes sense.
I figured out one time that the pump is driven at about 1.2 to 1.3 times crank speed on OEM applications. Maybe a guy could try 1.4 and see what happens.
Color me fairly clueless.