as most of the posters here have said, it will make a slight difference, but it shouldn't be significant.
i do most of my high altitude riding on a mountain bike with more like 40-55psi but i have hardly noticed a difference (last year did about 5 passes over 2700m). just check your tire pressure at the beginning of a ride and you should be fine.
and most of the passes in Europe aren't really that high. there are only a few places where you can ride a mountain bike over 3000m and even fewer on a road bike --- i'll probably be riding the GrossGlocknerHochStrasse some time this summer
and i agreed last night to do a roadbike race with a friend in July that goes over Stilfersjoch which is 2700m... (i rode it on the MTB last August)