Way up high stall speeds and such are related to the density (and therefore temperature) of the air. The atmosphere at the poles isn't exactly thinner, it's just more compact and thus the tropopause is lower as the air is much colder.
Roughly speaking, for the same indicated altitude the density altitude will be higher near the equator as the air is warmer and therefore less dense meaning your operational aerodynamic ceiling will be actually lower.
S.