I put in the Stewart pump and it appeared to be of high quality. My 2001 X5 3.0L original pump also had the metal impeller.

I would not worry about roughness in the impeller, though it would probably be more efficient if it was smooth these pumps are not exactly high tech. Pumps like yours are probably keeping thousands of BMW's cool. I don't believe the X5 normally has overheating problems, it is a well designed cooling system.