All Porsches that I am aware of use water pumps with composite impellers, which are known to disintegrate. Porsche engine modifiers/rebuilders recommend water pump replacement with OE (composite impellers) every 3 years, irrespective of mileage. OE water pumps are $300 (and up) and replacement every 3 years is crazy, IMO.I only put about 5K/year on it.
I use a Geba water pump with a cast iron impeller for my Porsche. Paid $65 for it last month. The OE pump was fine, but stories of early failure got me worried..
The prime reason for using an OE water pump with composite impeller is that should the impeller fall apart, or the bearing fail, the resulting shrapnel will not damage the block (only choke it with debris). A metal impeller would toast a $20k engine if its supporting bearing failed. One would have to be totally clueless or deaf to not notice a noisy bearing long before it failed and let the metal impeller contact the block.