What factors, besides the degree to which the tongue of the reed is open and the overall airtightness of the set, contribute to the efficiency with which a drone uses air?
L
Lemonsquash, I’m just a beginner, but my piping friends continually impress upon me the importance of leaks. There are many possibilities: tenon wraps, cracks, shutoff key washer and/or the cork thru which it slides, reed seating (tongue facing up!), ferrules, etc. etc.
Finding the leaks? Some use a fine spray of barely soapy water, others are horrified by this but blow smoke into their sets for the same purpose! I have no opinion, but prefer to try to find leaks as often as possible by just removing the reed (when checking for leaks in the drones themselves), covering one end of the piece I’m checking, and sucking. If any air is getting through, you have a leak. Without H2O or smoke there might be more guesswork nailing the exact spot, and in the former case you’d want to blow into the piece, of course. 
Leaks definitely affect drone efficiency. I’d love to hear more on this from the more experienced players, myself!
N, good luck to you
Sorry, L, I guess you already mentioned airtightness.
N, oops
I don’t know how well the technology would migrate, but these “tone enhancers” for GHB drones are intriguing:
http://pachome1.pacific.net.sg/~wongch/Maintenance%20Items.html
BC
Sometimes it may be that the drone reed has a bridle that is wrapped too tightly . If you can , Test a few drone reeds to see If you can find one that works the best .
I once had a bass drone that was not working , I did everything but throw the drone out the window , then I unpacked my whole kit , and lo and behold , inside the main stock drone shut off valve was a small piece of fluff , pocket lint , whatever , I removed it with a pair of tweezers , and the bass drone went off like a b 52 .. in a hurricaine .. lol . Meanwhile I had scraped and tweeked the bass drone till it was at its limit , lesson learned . the virtue of patience is patience .