Hi Nia and Rog
You are right of course, and that is why I haven't used nylon guy ropes for some time. They stretch far too much. I have used Spectra on my summer tent for some years now, and it works better, but the nylon fabric in silnylon still stretches in the cold. Ah well.
I found that thin Spectra can have problems. Stuff 0.5 mm thick has a breaking force of 150 lb, which should be fine, but you can't hold it, you can't put tautline hitches in it, and ... and it frets on my titanium snow stakes and breaks. Epic story there, which may appear here at BPL 'soon'. I lost 7 out of 8 guys in one 14 hour night.
So I am looking now at thicker Spectra or perhaps softer Dacron (with similar strength of course). The stuff has yet to arrive, so this is a work in progress.
My objection to bungee cords really only applies to extreme weather conditions. (Unfortunately, we just had that in spades - in the snow here in Australia.) The stretch then is just too much. And yes, the stretch lets the fabric flutter, which can be bad all round.
I don't have a really good solution, except to get dressed around midnight and climb out into the gale and reset the guys... I suspect a few of us have done that too?