Nope, the heat. A few hundred rpms can make a big difference in maintaining reasonable temps. There really is no need to run every lap like it's a qualifying lap, the pros don't, why should an amatuer?

I don't know anything about the Mishimoto Radiator but if it doesn't have a larger capacity than stock it really isn't much of an upgrade.

He says the thermostat has been drilled out but he doesn't give details. If it's just a 1/4-hole so the air can escape then it shouldn't be a problem, but if the whole thermo is gone then that is his problem. The water is moving too fast through the engine and not spending enough time in the radiator to cool down.

If none of the above applies then I would look at the underbelly. It could be holding up airflow in the engine bay.