Enabling CEM for the player's own aircraft costs about 10 FPS on most rigs.
Enabling it for every other AI aircraft would grind the sim to a halt, so the way the AI works is different: instead of monitoring the complete set of CEM parameters, they just follow a set of predetermined procedures and power settings.
This is done to prevent them from "cheating" without having to calculate everything and overload the CPU (this was said in one of Luthier's Q&A sessions some time before release).
In short, the AI doesn't look at gauges and decide what to do because decision making costs CPU cycles.
What the AI does is being fed a list of settings to use and following these checklists for their engine operation, dependent upon the phase of flight and maybe their skill levels.
So, on takeoff they will get to the appropriate manifold pressure/boost and RPM, then they will reduce power to climb or cruise, then they will go to combat power once they spot enemy aircraft and so on. They don't know about or even "feel" the limits like a human player does, they just follow a set of restrictions that prevents them from getting an unrealistic advantage.
Somewhere along the list of those restrictions there needs to be added a bit of extra code that follows this logic:
If AI Level = rookie then warm-up time = X seconds +/- 5%
If AI Level = veteran then warm-up time = Y seconds +/- 2%
etc,
and everything would be fine
