Robert,
   Very good question. Personally I would remove the old pump and compare
the clearance with the new water pump in your emergency spares. What? You
don't have a spare, German built water pump? Oh the humanity.
    The few old pumps I removed had a visible amount of wear between the
impeller blades and the body of the pump i.e. increased clearance. The
larger the gap the lower the output pressure and hence, higher operating
temperatures. I find it very interesting that metal (of the blades) could be
worn down by coolant. Thought provoking question: Where did that metal go?
   I, personally, from experience, would only use German, as opposed to
Italian made pumps.
Ken Lewis
86 Transporter Crew Cab
60 356 T-5 Porsche
http://neksiwel.20m.com/
Vanagon tech, pixs, O2 monitor gadget