[a90]

Our cars (M) are way overpowered for normal street usage, the amount of power needed for the airconditioning is about 10hp max on a hot day.

From that perspective, a stock M5 will have 550hp 'left' on a hot day for driving through the mountains. Seems enough and should not lead to problems.

If you have an older car with old technology airconditioning (less efficient, uses more hp) it might lead to issues when you only have 100 / 120hp or so. In that case, yes take that into consideration on a hot day in the mountains.