Wes
At idle speeds the thermostats control the operating tempurature of the motor. At around 2000 rpm the poppet valve begins to open. It acts as a bypass to the thermostats and allows hot water from the heads to dump out through the leg increasing cooling. Its basically a spring loaded plastic plug the thickness of your finger and is located bottom right side of the motor under a cover.
If its sticking because the rubber "seat" has swelled, the diaphram is damaged or even the cover gasket has ballooned out to cause a restriction, it will not open properly causing a a hot condition possibly an overheat condition in the 2000rpm range. At idle all seems fine because the thermostats are doing the control, at WOT all seems fine because the pump pressure is overcoming the problem and forcing the valve open.
With cooling issues you should always go to the pump first but if your confident in the pump and and thermostats the poppet valve is the next suspect. When doing the pump housing and impeller did you replace the gaskets in particular the one under the stainless plate and did you run a bead of silicon along the 'wall' immediately behind the pump housing?
New components might do the job but there are a hand full of Mercs that for some reason just always have this issue as factory standard and need a tweak.
All this said be wary you can be chasing something that might just be ordinary operating condition but I suppose you have gone so far in chasing this thru, the poppet is the logical next step and is a relatively inexpensive job for peace of mind.
Cheers
C.