When one talks of adapting to an altitude, what also matters is how fast one is ascending. The critical part to note is that having a higher haemoglobin level does not mean one is better equipped at alititude.
Typically it's safe to climb around 300m per day.
Coming to the question of living at altitude and the possible advantages of acclimatisation as against lowlanders. There are many studies which compare people living at a higher altitude as against those living in the plains. There are also studies that compare the physiological adaptation of different highlanders. And the interesting part is that Tibetians, who live at an altitude of around 4500m have lower Hemoglobin levels than those living in the Andes!
Tibetan males had a significantly lower mean hemoglobin concentration of 15.6 gm/dl compared with 19.2 gm/dl for Aymara males, and Tibetan females had a mean hemoglobin concentration of 14.2 gm/dl compared with 17.8 gm/dl for Aymara females. The Tibetan hemoglobin distribution closely resembled that from a comparable, sea-level sample from the United States, whereas the Aymara distribution was shifted toward 3-4 gm/dl higher values
One factor that was found which allowed Tibetians to have a good adaptation to altitude was the rate of ventilation and the rate at which oxygen was getting delivered.
In addition, Tibetans may have a second biological adaptation, which expands their blood vessels, allowing them to deliver oxygen throughout their bodies more effectively than sea-level people do.
- Adapting to altitude depends not just on hemoglobin content.
- Given that the studies show that highlanders themselves have different ways of adaptation, mid-highlanders would surely have a higher density of hemoglobin or a possible higher rate of ventilation (the rate of this might not be linear).
- Rate of altitude gain decides how much your body will adjust to the altitude.
- Studies mentioned in the links provided show that lowlanders do adapt to higher altitudes by both ventilatory and hemoglobin content changes.
Links for additional read: