LOL well it really is noticeable.
On a trip to North Cascades a couple of years ago I had to purchase Heet locally, of course due to air travel, instead of the Klean-Strip DA ('marine fuel' which is a methanol/ethanol blend) that I normally use. Heet is essentially pure methanol and I noticed, sure enough, that I needed more of it to boil water even at somewhat higher elevations (3000-6000ft) than I do locally (typically 500-1200ft) with a higher boiling point. I wasn't being particularly careful measuring it out and darn near ran out of the 12oz bottle even though it was only a 4-night trip. Lesson learned.