It is a combination of the added richness AND the cold dense air that the engine is receiving. A gas engine is an air/fuel pump, as the amount of air AND fuel the engine pumps in increases, the power increases proportionally.
You should not drive ANY engine hard when it's cold, you are drastically decreasing engine life. If the engine is cold enough to run like this, the oil is nowhere near warm enough to circulate and do it's job properly at high RPM. I don't care what kind of super oil you use.
In race engines it is not uncommon for an engine to be lost after cooling down during a red flag period and then run hard immediately after restart. Cold oil is incapable of doing it's job properly.