Can't help you there. I grew up at a lower elevation of 4500 Feet and -40 C winters (Canada), and I do have asthma. But I don't recall having problems with exercise except in cold air. I have no idea what my O2 levels are, I do believe it usually is around 98%,based on what I know from being in surgeries. I do know that my sea level relatives always complained about the thinner air where I grew up, and they required naps till they were used to it. Not sure if it was a psychological thing for them though.
Also, I found this comment at a high altitude hiker site (I'm the first to exhibit altitude sickness when we go higher, yet still have sats 5% higher than anyone):
A misconception of the acclimatization process is that the body returns to its sea level condition or that the hypoxia of high altitude can be nullified by acclimatization. This is not the case.
No, a reading of 99% O2 saturation is NOT normal at altitude. Normal readings are 90 - 95%. I have been working with a sports medicine doctor on my poor aerobic ability, my high O2 levels concern them, though they don't know what could cause it.
actually, your lungs are optimtimal. Under 95% is considered a problem, and an investigation should be made into such low numbers. Even at high elevations a persons body over time can adopt to the thinner air, by making the lungs work harder, which is nothing wrong with that.