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The rear of the water pump housing in the front of the engine case. The impeller blade has to sit very close to it (perhaps a couple thousands of an inch) in order for the pump to actually move the coolant. Eventually, all water pump bearings wear, and the shaft starts to wobble a bit, when that happens, the gap between the water pump impeller and the cases starts to get very tight and eventually the impeller makes contact (which is also why when a composite impeller wears, it wears at the outer edge). When it does this, there is often no noise as the sound is dampened by both being enclosed and submerged in coolant. So you have no warning anything bad is happening until you suddenly find an intermix problem.
Porsche is not the only one using composite impeller pumps because of the proximity to the alloy engine housing. A metal impeller is harder than the housing, so the housing will lose the argument. A composite impeller is softer than the housing, so the opposite happens.
Automotive engine design is always a matter of trade-offs. With a metal impeller, the risks are simply higher.
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“Anything really new is invented only in one’s youth. Later, one becomes more experienced, more famous – and more stupid.” - Albert Einstein
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