Tiggers, it's really very simple.
If the water is returning hot from the skin tanks, then they are too small. Simple as that. It's not a pump problem, because if the pump were too slow, the water would have extra time to cool, and would therefore be cooler, not hotter. And if the pump is too fast, and the tanks adequate, the engine would not be getting too hot.
If the water is returning cool, and the engine is still too hot, then it COULD be a pump problem, in that the water may be being pumped around too slowly to remove all the heat. If that is the case, the water coming out of the engine will be very hot.
It could also be a thermostat problem, if the 'stat is not cutting in until the engine is too hot. Measure the temp of the water. Above around 75 degrees it should start to be diverted to the skin tanks.
It could also be a blockage/air-lock in the skin tanks. I had that problem with my JP3. Water was being diverted at the right temp by the stat, but the air-lock prevented much of it from getting through. So not enough was going through the skin tank, and the rest was being forced through the thermostat and back round the engine. Bad for pump, pipes and stat. In my case, the problem (an air-lock) cleared itself on a long run with the engine working hard.
Surprised you don't like JP3s. They are the smoothest and quietest of the JPs.