Since the body is going to produce more EPO and red blood cells your performance will probably increase in the next weeks while the pressure is constant. However, as soon as you return to the sealevel you should notice improved performance to the prior level. Are you actually diving at that altitude right now?
Exactly; although you are likely to have a reduction in performance due to the lower partial pressure of o2 your body will compensate in several ways including changes to the components of blood. Give it at least a couple of weeks and you should see a positive result in performance.
Bear in mind that at sea level where the partial pressure of oxygen is higher - carbon dioxide levels in the blood are the main driving force for the desire to breath (in situations where people want to suppress their reflex to breath - hyperventilation is sometimes used - as such CO2 levels in the blood reduce and the desire to breath decreases).
However where altitude is enough to cause significant hypoxia - you are likely to have a reduced level of CO2 in your blood. This is due to significant hypoxia triggering hyperventilation. Unfortunately hyperventilation doesn't have a big effect on the levels of oxygen level in the blood (as the fraction of inspired oxygen is constant). However with hyperventilation you will quite literally "breath off" more CO2 and therefore a reduction in blood CO2 will be seen.
This is true for the initial time period at altitude. As days and weeks go on, as mentioned earlier, your body will make physiological changes to increase oxygen to organs. As this improves the previous hypoxia induced hyperventilation will decrease - this will lead to blood CO2 levels rising back towards similar levels you would have had at sea level.
Please note in this context I have used the word hyperventilation to describe an increased rate of breathing compared to what would be normal for you at sea level (I do not mean a respiratory rate of over 20 per minute).
Also at the altitude you state the level of hypoxia will only be mild. As such the rate and extent of physiological change would no be as dramatic as being at 5000meters - hence why I said give it at LEAST a couple of weeks!
