Those who are saying that having a power plant with a larger capacity than is needed reduces heat signature/temp (and has nothing to do with heat efficiency) are correct. I am using an A4 power plant in my Cobra and had about 10% extra power than I needed and my base temp was around 42. As I made money I upgraded all my power consuming modules and added other modules until I was at 101% power consumption, and my base temp now idles at 45-46. I not really sure why this is the case, but this is real empirical data and not theory. Note this is using the same power plant (i.e. the same heat efficiency).
That's because you added more heat due to larger power consumption. That's not what people have been saying. The only way to test is to use the same modules and switch back and forth between two power plants.
In other words your heat went up because you went from say, 9MW to 10MW power consumption - not because your power plant went from 90% to 100% usage.