Nitromethane doesn't actually burn, it really only carries oxygen into the combustion chamber so it's basically a chemical supercharger so I would imagine (this is just reasoning, I don't know it as a fact) that more nitro would either be wasted (assuming the engine's already burning fuel efficiently) or would raise the temp rather than lowering it (due to more fuel being burned if it was running inefficiently at lower nitro content).

The temp rating of the plug won't have much effect on the running temp of the engine either. Plug temp is a way of changing the timing of the engine... a hotter plug will cause the fuel to ignite earlier in the cycle (advancing the timing), a cooler will have it fire later (retarding the timing).

If your glow engine's overheating there's really only three possible reasons...
1: The cooling fins are clogged with debris. Clean your engine.
2: The engine's encased in a cowl or similar which does not allow sufficient airflow. Allow sufficient inlet holes, and outlet holes 2-3 time as big as the inlet to avoid a high pressure area.
3: The most common (and easiest fixed) reason - your engine is running lean. When setting your mixture, tune to the peak RPM then detune the engine rich by about 1/4 turn. This allows for the engine to lean out in the air due to lower air pressure at altitude and the prop "unloading".

FYI, an engine will always run hotter when bench tested than in flight, due to the lack of "blast cooling" from the aircraft moving through the air - the same as if you let your car run long enough without moving it, it'll eventually overheat whereas driving it will keep it cooler.