There's an old TOSEC demos-executables floating around that I, and perhaps others, have. But anyway...
Ideally, everything would have to be rezipped using something like trrntzip.exe (like they do at Underground Gamer) but even just a dat of the single demo files would be great. Assuming best compression, they should be relatively close. And if it shaves off even just a little, that would mean a better seeded torrent and people could jump in and start, already having a portion ready to start uploading to others.