actually he hasn't changed the heating system that much.

Lets assume the koyo is a larger more efficient radiator than the stock unit.

He has increased the coolant capacity of the cooling system. The increased capacity gives him X amount more water.

The unit is more efficient than the stock radiator, so water is cooled faster or to a lower temperature.

Both of those things would mean his car should operate at an overall cooler temerature.

But since the thermostate controls the radiators use....

If the radiator can cool the water cool enough that the thermostate closes on the freeway, he will probably see the car operate on city streets and spirited driving at similar temps as his old unit.

He will just have more heat capacity.