I have the March 809 with the 815 impeller and I've never buy the 809 again. What a huge difference in priming and flow after upgrading the impeller!
I also have a Chugger stainless head with the larger impeller. I think they come with an impeller almost the size of the March 815, and the impellers are interchangeable between brands as well as I've done it.
The March 809 although becoming a standard is clearly not the best for our hobby. The 815 or Chugger works better than the 809, and the center input model with the larger (3/4"?) input I've seen work even better, and would be the one I'd buy next if I needed another pump.
My installations are textbook proper per the manufacturers so the only variable has been the impellers. Don't buy the 809 if at all possible.