Actually, I think that poor scratch resistance of subbed films (both 3M and Dupont) are the material's Achilles heel. The Dupont film that The Formulary sells has minute scratches. They're almost impossible to avoid. I'm getting much better at handling film, but (so far) it's a guarantee I'll end up with a scratch somewhere in any given batch of finished film -- usually from the final cutting-to-format stage. I'm pretty religious about wearing cotton gloves, so I can't speak to grease resistance. I can tell you that you don't want to sneeze on the stuff .
The good news is that the scratches are essentially microscopic. (More info: http://www.thelightfarm.com/cgi-bin/...tent=28Nov2011 )
One issue that I'm still working out is that the 3M film doesn't seem to hold dry emulsion as well as the Dupont if the coating temperature is too high. I've got the right temp for my emulsion figured out, but I'm not motivated to test, test, test all parameters, just to obsessively blog, so I'm hoping others will join the Light Farm 'research team' and share their findings. (A girl can always hope.)
The 3M vendors I talked with all would do custom cuts for a price. I never asked about sheets. I think the film is only available in rolls. I was able -- and quite happy-- to take remnant end pieces in various widths -- for both the thin and thick films. I got a great deal that way. My only requirement was that no one roll weighed more than 50 pounds or was wider than 36 inches.
You're right about hydrophilic subbed PET being a game-changer. Seriously, there is now absolutely nothing standing in the way of really, really good diy film, plates, and paper (except of course, the exact same things 'standing in the way' of all non-instant, non-digital photography -- enough time and space to create.)