I don't remember if the pumps compared was for an Mercedes inline-6 or V8.
However, I only paid $89 from the dealer for a M103 waterpump about 2-years ago.
Search the Mercedes archive on the Ritter-Easley list and you will find the person who experienced the higher temps with the aftermarket waterpump. I believe it was only during IDLE that the flow wasn't sufficient.
Again, with the factory warranty and labor covered, I still think it's smart to get Factory parts on some items.
1988 360TE AMG