After re-reading this page, post #68 has a hint, i think.....port engine raw water pump delivery is right on the edge of being adequate where starboard appears to have some margin. I'd would be inclined to measure the output from each pump, at 1000 and 1500 RPM and compare them. I'd bet the port side is less. if so, I'd check out the cam in the pump for wear...and if it uses the sherwood pump, with the graphite bushing, make sure that is tight, too.....
Also, the cooling water with take the path of least resistance thru the block and manifold. based on the pictures from earlier, there appears to be two sources supplying cooling water to the exhaust, 1 from the heads, at the intake manifold, and one from the tstat housing to the elbows....I'm thinking the path from the head to the exhaust has got to be bigger (flows easier) that that from the tstat to thru the elbow, if for nothing else than the cross section of the elbows...descaling wont hurt anything but I'm not thinking its going to matter much....