I've no problem with leaning off the mixture at any altitude after take-off if you've got a fuel flowmeter and CHT gauge!
The military used to fly the Bulldog at 2600, best power mixture, MAP as req'd for most of the time - mainly for performance and 'single lever' methodology for the students. But it drank fuel.... I once flew one from Abingdon to St Mawgan with the prop back as far as I dared and MAP up just below the point of over-boosting with the mixture leaned as far as possible. The aircraft flew just as well at the same speed as my gas-guzzling chums in the formation - but I landed with 3 gallons more than they did! BUT it was necessary to keep a very close eye on the CHT and to select a richer mixture periodically.
Pity we'd stopped teaching the students how to set the engine up for long range economic cruising - but they were all training to be future jet pilots!