I suspect cats are like people... Sometimes at high altitude they find it hard to breath due to the thinning of the air.
For most cats though this won't be much of a problem as they will conserve (Sleep)
My pair has traveled with me across the mountains a number of times I've seen 7,000 8,000, 9,000 and IIRC 10,000 altitudes and they did not have a problem (nor did I)
This matters,, Wife and I are NON-Smokers