IIRC the reason for higher engine temps for economy are:
1) the higher the engine temp the lower the friction (mainly due to design clearances and pumping of fluids),
2) also the better the vapourisation of fuel prior to combustion leads to more complete combustion (a more homogenious mixture)
3) and the lower the differential between combustion flame temperature and piston/cylinder/combustion chamber temperature.
The chamber temperature is an interesting one as the heat released to the metal by the flame (and hence the loss of energy to the coolant) is actually less the higher the engine temperature. This means that more energy is available to expand the gasses in the cylinder and propel the vehicle.
Ideally the engine would run at the same temperature as the combustion flame, so there would be no heat rejection to the coolant and therefore no energy loss & perfect efficiency
