All metal impellers are not created equal. Looks like the aftermarket metal impellers have a significantly thicker vane than the GT4 impeller and the stock composite impeller, would have to spin it at the rpm the pump sees at 6k+ to see if it starts cavitating earlier or later than the stock or GT4 setups, but something to think about
GT4:
Stock replacement from ECS:
Genuine BMW stock replacement:
