I will go over hight mountain, about 11000ft over sea.
Has anyone already flew to these heights? Are there differences in flying at much heavier heights?

Just keep an eye on the battery level and temperature and you should be good! Test it with a some close flight and keep it in LOS.

Thinner air at that altitude will require more power to keep the AC aloft and maneuver.  So expect lower battery life.

I fly above 14,000 almost every weekend. The Mavic does fine. There is a power reduction but it's hardly noticable.

Thanks to everyone for the answers, next week I will go.

I would recommend you to take care of the temperature changes and battery performance.

watch out for the wind - fly mavic upwind from home point if it is blowing.