Which water pump are you talking about, and for which car? $89 for a rebuilt waterpump on a V-8, or the inline 6? I'm asking because a rebuilt on my 928 S4 is only $99, but it is a much simpler casting than the water pumps on the inline 6 motor. The water pump for the 928 V-8 bolts to the front of the block, whereas the one for the E320 bolts to the side of the block. I don't think we're talking about the same water pump.
Put the aftermarket pump vs MB pump into a test-stand, and show me the two performance curves for circulating the coolant in the cars. Saying the impeller on the MB original is shaped differently does not automatically guarantee its performance is better. From a hydraulics standpoint, I'd like to know if there is a significant differnce. You can achieve the same flow and head from differently shaped impellers, albeit your efficiencies will likely be different.
Certainly MB wants their customers' business for components on their cars. That doesn't necessitate that they will outperform every aftermarket component that is sold, since a waterpump typically is a relatively simple component (casting, impeller, bearing, shaft, etc.)
95' E320 (95,000 mi)
88' 928S4 (81,000 mi)
96' Ford Contour (139,000 mi)
86' Jeep Grand Wagoneer (126,000 mi)