M14P
Yes one needs to lean at high elevations etc but then the engine is nowhere near developing full rated hp. If the airfield is at say 7000ft then the full throttle hp may be only 60% of max rated. So one isn't actually leaning above 75% power.
There are various articles on the lycoming website - sorry I don't have the URLs handy but could dig them up.
Not a problem in the UK
What happens with a turbo engine (which would develop max rated hp at 7000ft) I don't know. Do they need leaning at takeoff?