Here is a real world data point from a metal impeller heretic. I changed the original water pump in my 03 S about 4 years ago at around 42,000 miles and had one with a metal impeller installed. This May at about 66,000 miles the bearing started to fail on that water pump. When the bearing failed, it started leaking and the light in the temperature gauge started blinking. The air temperature was in the 90's and the water temperature on the gauge stayed just above 180. I drove home about 50 miles and then had it flat towed to my Indy specialist. He said that he'd never seen a water pump fail that quickly, and his parts supplier replaced the pump for free under warranty. He also uses the same metal impeller water pump in his 987 Cayman S with a 3.8 liter engine. My point is that in this instance, there was a clear indication that I had a problem and it was addressed well before the impeller came in contact with the engine block. The thing about a composite impeller is that pieces of the impeller blades can break off before the water pump bearing fails, those pieces can lodge in the heads and cause hot spots and cracks in the heads. That will happen at some point due to the impact of heating and cooling cycles. Either metal or composite, they both need to be replaced proactively.
I love my car, and I'll be the first to say that Porsche got the vast majority of things right with it. But keep in mind the composite impeller water pump is brought to you by the same people who brought you the IMSB, headlights with lenses that melt, sticky black interior plastic and glued in GT 3 cooling system fittings that fail.
