Just from a similar experience with a different auto maker, the 6 blade will move more water at higher speeds due to it's ability to more completely fill the impeller area.

I wonder if there is a 6 blade with a plate like the one on the left out there? I would think it would be as good or better due o a thinner wall profile.

But maybe I am thinking the speeds are higher than they truely are in these cars. Does slowing the water pump speed help?

Dave