Interesting thread. I had heard about these expensive electrical water pumps from my Indy. My 550i doesnt have one ,but my 2011 e93 does.
Regarding the plastic impeller, these are very high quality pieces. Being in the plastic industry, this is very common and the engineers love the advantages of weight and ideal vane design for pump efficiency. Stainless stamped/welded impellers offer poor efficiency and cast are heavy and typically have a much rougher surface finish.
Hell, airplane bodies are 'composite'.