I am not yet on the GHB. I have played my smallpipes at 6,288 ft. in Sargents Purchase, New Hampshire, as well as at 21 ft. here in Boston, Massachusetts. It might be my being a total novice, but the differences seemed to be minor. At the higher elevation, the reeds seemed to have less back pressure and struck in earlier than I expected. Blowing up the bag for an extended period seemed tougher, but the bag usage seemed a bit longer before refill as the arm needed less pressure to keep a consistent tone from my drones. I seem to need more blow and squeeze at the lower elevation. I did find the tuning to be very different at the two elevations.
I noticed that I needed to use my lighter pressure reed in the practice chanter while on the mountain to not tire out quickly.
The nice part about gigs with an improv band, is getting paid to practice.