Graham, in addition to the knot data, Tom's paper also contains results on rope strength after repeated flexing. The flexing data might provide some indication as to how the rope would perform after normal 'wear and tear' (poor choice of words!).
Nylon ropes have 90% knot strength, and flexing does not degrade them at all.
Technora and kevlar have 40% knot strength. They initially degrade very quickly under flexing, and the rope strength subsequently seems to plateau at 40% of the initial stength.
In real-world use, technora will have a 2,000 lb breaking strain (5,000 x 40%), and a poor knot strength. For comparison, the nylon rope tested had a 3,000 lb breaking strain and excellent knot strength, so the nylon rope is quite a bit stronger.
Re is technora used for rappelling? It seems to be used for emergency rappels by the military and possibly by firemen. Some himalayan mountaineers use 5 mm cord for rappelling, though I'm not sure whether that's technora, kevlar or spectra. Incidentally, I doubt that the NYFD uses spectra, because it has a very low melting point, lower than nylon (220 F?). For comparison, technora is around 800 or 900 F.
Re why not use 7 mm or 8 mm rope? 8 mm rope weighs 37 g/m , 7 mm weighs 31 g/m, 5 mm technora weighs 25 g/m . So 7 mm rope is similar in weight to 5 mm, while 8 mm is quite a bit heavier. I haven't seen any manufacturers endorse the use of 7 mm for rappelling, and I wasn't sure whether the sheath on 7 mm accessory cord was sufficiently robust. It seems that Edelrid will soon produce a '7 mm rap line', so that answers my question.
Re descender for thin cord: the Petzl Pirana provides a lot of grip, so it might be worth a try.