Some thoughts, not directly related to the question at hand, that might be interesting to people who fly at higher elevations.

Flying in Mexico City (7,000ft + ASL) this is a subject that comes up quite a lot. A general rule of thumb is that we loose about 20% of power/thrust compared to SL flying. This obviously varies by engine, plane setup. So we set out to reduce weight by 20%, or use larger engines without increasing weight.
We also use more wing incidence and fly nose heavy to help penetration and use higher nitro levels for glow engines (advise from Quique Somanzini at an acrobatic clinic we held here a few years back).
The availability of lightweight gas engines has produced some interesting possibilities for high altitude flying. The typical SL set up at 16lbs for 50cc IMAC planes with a DA 50, for example, has been pretty disappointing. At 100cc OK but not great. So the challenge has been on to reduce weight and or find more power.
In am currently using a 52cc FTL in a 35cc size Aeroworks Extra 260 weighing 14lbs. The FTL is an inexpensive, light weight, engine and turns a Mejzlik 20x10 at 7,000 RPM. The CG is slightly nose heavy and the flight performance is great.
My next project will be an Extreme Flight 50cc Extra 260 (hopefully at 15lbs) with a RCGF 65cc engine that should turn a Mejklik 22x12 at around 6,800 RPM.
Other guys are using BME 100cc engines in 75cc sized IMAC planes, which produce very acceptable performance at about 20lbs.
Obviously I am referring to precision acrobatic (Pattern) setups, but in general all categories and sizes can benefit from these guidelines for altitude flying.