i can't accurately compare the water temp between stock and HDi due to the outside temp and traffic condition may different.
for the cooling system, i have nismo thermostat, full alloy radiator, breath tank, Motul Mocool, just missing scotty's mod. i agree scotty's mod could stabilise water temp, but less effect when stuck in traffic on hot day? it is all good if my car is running above 30-40km/h all hot day, just problem when stop on idle, especially when aircon is on. if i go hardly even though at very low driving speed, then water temp will go down a bit. it seems the stock noisy fan or waterpump can't flow enough air/water when idle.