Coolant flow

Saying higher coolant temps looses power as an absolute is simply incorrect. It may or may not.

It will depend on what results from higher coolant temps. If it is a 'looser' engine that has increased the internal clearances, then there is a power gain from less friction. The oil thins out as temps increase & can reduce friction, increase hp. The power output of an engine like a 383/440 is far less likely to be affected by coolant temps because there is no coolant in the intake manifold to heat up the incoming air.