the R-44 has power well beyond the MP limitations
You need to add "whilst at sea level" to this statement.
Several years ago I was doing some pinacle approaches with a very experienced instructor to a peak at 5500' and around 30degrees (it wasn't in the UK :-) I was amazed how the power just ran out exactly like the limit charts said it would.
One of the reasons for derating the engine is to gain acceptable high-altitude performance without turbo- or super-charging the engine, both of which result in reduced reliability.
It's actually quite easy to work out mathematically, no test pilot required.
The Lycoming IO-540-AE1A5 engine in the R44 Raven II develops 300bhp full throttle at sea level. The maximum
continuous power allowed is 205bhp - so during normal everyday operation assuming you observe the MAP limits this is all you will use and you'll be operating somewhere below full throttle. You can pull a bit more for five minutes but that's basically all you've got.
A normally-aspirated piston engine loses approximately 3% power per 1000ft increase in density altitude.
If your pressure altitude was 5500 feet at 30C that gives a density altitude of 8620 feet, and full throttle engine power is reduced to approximately 209bhp. So now you're operating on the same power limit in terms of available bhp, but this time at full throttle and you certainly do not have "power well beyond the MP limitations".
I designed a spreadsheet to examine the effects of weight, balance, and density altitude on flight performance during my training to help myself understand what effect the different parameters had. Anyone interested can download it
here.