So I had been having some cooling issues (outlined previously here, and here). Last week I was able to track down an OEM, Wahler brand thermostat. Swapped it in over the weekend and lo and behold, the issues were solved! Merc ran between 82-88*C, holding pretty steady at 85*C the whole time. Temp really only reached up higher (maybe 90*C max) when coming back to traffic speeds after being at freeway speeds, and would drop back to 85*C shortly after.
Thoughts on the thermostat though: I boiled both it and the old T-stat and they had nearly identical start to open/fully open temps. The Wahler one though did have a larger diameter valve, as well as a different shaped flange where the valve met the body. I guess the shape and size difference was enough to impede flow on the aftermarket t-stat. Other thing that seems odd is my mechanic had done the first t-stat swap, and he usually uses high quality parts, so I would’ve thought the thermostat he had in would’ve been correct, but it had the same issues as the aftermarket one.
Anyways, glad that that issue finally (hopefully) seems resolved. Now I can turn my attention to any other needs the ole girl throws at me.