As ugly as that cast impeller looks, I would not doubt it may pump with more efficiency. Smooth, pretty surfaces don't mean better pumping, in fact rough surface might be better. I would be more concerned about the bearing and seals and I'd rather have the metal impeller than the plastic one since the reason they fail is because they are plastic.
EDIT: whats up with this forum? I can barely follow most threads. This note just got stuck in a weird place. Not to be a whiner, but is someone on top of this problem?