TV performance and elevation

Mr Biggles wrote:
Do manufacturers take elevation into account when publishing towing performance numbers ? I live in Calgary, elevation 3600 ft. After towing our TT for the last 3 years , I would not want to tow much more than 6000lbs with our 5.0 F150 though the mountains. A forced induction engine might have been a better choice, but even then there's a power degradation. I realize that elevation does not affect frame strength or braking, but it definitely hurts power.