I'm guessing your temp sensor is in the return flow back to the engine, after the radiator? Otherwise the minimum temps would be greater than the thermostat opening temp. (unless it is drilled or faulty)
My car saw 122 degrees water temp at Sandown on a 30 degree day, after only 3 laps. This is taken at the outlet of the head at the back of my VQ. If I measured the temperature at the radiator outlet it would be much lower obviously, and I wouldn't be so worried. I still have to try blocking the sides to force air through the coolers, that should help me. Otherwise I may have to resort to a water spray kit I guess.
Running 500hp on a 2.5L is going to generate much more heat than a 500hp NA 5L imo.