All i know is that takeoff power or emergency power is called that way for a reason: it is to be reduced to lower levels under all other regimes of flight.
Otherwise they would just call it full power and be done with it.
The mustang's 67" of MP is equivalent to the Spits +12lbs, the 109's 1.45 Ata and so on and so forth, take your pick, in the sense that they are not meant to be ran for eternity because things will start to break. Maybe not on this sortie or the next, but definitely something will give after a few missions, especially if i push it that way on every single sortie and the mechanics follow your reasoning of not inspecting it afterwords
As another example, for later mark Spitifires like the Mk.IX it was advised to take off with a mere +9lbs no matter if it could do +12, +16 or +25 and that's a pretty critical phase of flight in terms of power reserves in case something goes wrong.
If they didn't slam the throttle to the stops on takeoff that's telling me that full power at low airspeeds was a combination for insufficient cooling, overheat and eventual engine seizure if the oil dissolved. True, this is for later mark Spits with a higher power output, but these also had an extra radiator to help with cooling which our in-game early Spits lack, so it's more or less a trade-off.
All that is enough explanation for me to convince me that operating limits are there for a reason. I want a difficulty setting that imposes penalties if i exceed them, that's all. If you don't like it, feel free not to use it.
However, the majority of people in the community won't stand for implementing changes to the FM/DM that have all other aircraft adhering to some kind of limits while the Spitfires suffer none and it's not even for balancing reasons. It's because it's common sense to assume that emergency and takeoff power are named that way for a reason.
Can we please get back on the topic of how such a game mechanic/feature could be implemented? If you want to continue debating if +12lbs classified as emergency power, feel free to start a different thread about it, you're just being off topic in this one:
The current thread is not about "what can the Merlin run with impunity?". The topic is "how do we punish the player that exceeds what the engine can reasonably run, if he chooses to enable the relevant difficulty/realism settings". It's about ALL engines, not just the Merlin.