I never even knew that there was such thing as a specialty marine pump shop! They said that the pump, being old (probably, I don't think it was replaced when the engine was rebuilt), could well be worn out from many years of use. Or have a poor seal on the front or back of the pump. An additional variable --a few years back, I replaced my pump cover with a SpeedSeal (www.speedseal.com) since the tiny bronze screws on the stock Volvo impeller cover sheared off, and they're very tiny and easy to ldrop and lose anyway, so it seemed like a logical, cheap and easy upgrade. So, this might be affecting pump performance if there's a bad seal.

This specialty pump shop recommended doing the raw water flow checks first, which are easy to do; and if the checks point to the pump, they said it's a very easy matter for them to determine whether the pump is worn out or not and pumping according to spec. The pump is easy to remove; so, having this assessed, if needed, would help eliminate that as a concern. I'd never realized that they could wear out --they look very simple and reliable as long as a good impeller is fitted...but...

Anyway, I'll do a few raw water flow checks first, then on to the next steps...thanks for the tip, D'Arcy.