I do not, and am not endorsing exceeding limitations.
None of what follows addresses the very valid concerns related to transmission overloading. Do not discount those. As for the engine....
The manifold pressure on a gasoline aircraft engine is partially a measure of how much power is being developed, but more to the point, how the power is being developed. If you have high manifold pressure, you have lower RPM, (because the rotor is limiting it). If the RPM were not limited, it would increase, and the manifold pressure would decrease. High manifold pressure and low RPM is the combination needed for detonation. The manifold pressure limit is there is part to provide a margin of safety against damaging detonation.
Very minor detonation can be tolerated by the enigne for very short periods of time (the margin part), but even when the operating conditions which caused it are removed, it will continue for a while, and continue to cause damage. And before someone jumps on me for how I know, I have successfully conduted Transport Canada testing for detonation margins in aircraft engines. I have seen pistons from enignes in service with a 2" diameter hole in the middle, from detonation. Lycomings in particular, are intolerant of metal chunk contamination inside the case. It is the possiblity of damaging detonation that is the reason we do not run constant speed propeller piston engines "over square" in airplanes (unless their turbo'd), and why we don't run lower octane fuel than specified for the engine.
The damage to an engine from detonation may show up at overhaul, long after (if you're lucky) or may show up minutes later as that hole in the piston. That will be very dramatic. It could show up as that hole in the piston for the next guy who gets near the limits 'cause the last guy exceeded them. Unlike a car, the pilot will have zero awareness of detonation occuring until either the engine disintegrates, or a mechanic later tells him it nearly did. The helicopter is not instrumented to tell the pilot detonation is occurring, and he cannot hear or feel it. Trust me on this.
As I said, the manifold pressure limit gives a margin of safety. It is not the pilot's business to know how big that margin is, or try to work into it. As is the case with nearly all limitations in aircraft design and certification, a margin is given. Approved testing should be the only time that margins are explored, other than the previoulsy mentioned life threatening event, which is then followed by honest reporting and inspection. A piston engine overhaul shop knows how to inspect for damage resulting from possible damage from detonation.
Those who wish to know more about this subject would learn well by reading FAR 27 and 33. Then you'd understand why we place operating limitations, to tell pilots how to stay safe!
It was well said that "regulation is for the guidance of wise men and the obedence of fools". It applies here.
Pilot DAR