Its late and I'm a bit pressured for time, so I'm not guaranteeing that I've haven't made a mistake.
But my reading of the performance chart in the Engine Operators Manual is that 40 in Hg manifold pressure at SL yields 100% rated power (200 Hp). But 40 inches at 12,000ft yields 215 Hp. So, for a given manifold pressure the engine delivers more power at altitude. So, its got to be about inlet temperature and / or water vapour content (humidity). Humidity has a significant detrimental effect on power.
The performance graph also suggests that the engine can only maintain 40 inches to 12,000ft and after that it drops off, crossing 200 Hp at 14,000 ft where it makes about 37.5 inches.
From experience, this is a bit academic, because you're likely to tun into TIT and CHT issues at those altitudes and high power levels. As the air gets thinner, it also becomes a less effective cooling fluid.
The engine operators manuals are relatively cheap and worth having. On long flights it can be interested to pull them out and see where the engine / airframe performance plots. I'd recommend buying the books.