I love it when the man himself asks us any question, and a question about backpacks is probably my favorite.
Ryan I think for a pack to be justified to be SUL it should at least start off as a pack that was intentionally designed for SUL hiking and not as a lite day-pack.
This depiction between the 2 should also be considered to be for 3 season only so what comes below will not apply.
90% of all the SUL lists I see posted (sub-5 pounds) DO NOT INCLUDE AN INFLATEABLE PAD. This means that all but the more compact half sized pads should not be a problem if they were not able to fit, unless the pack was specifically designed for something that size to be attached on the back as the cushioning.
SUL should be limited to a size bag that will include a sub-5# base with 3-5 days of food. Any thing longer and you are getting into the packs that are intended for comfortable storage and a 25+ pound max weight load. Besides, at best, these 25# load packs should only be considered to be UL. Even the lite 2500-3000ci packs are way too big for SUL. Most people going SUL find that even the with the considered small 2000ci G6 pack, they have PLENTY of room.
I don't see a sub-5# base weight with food and water added coming anywhere near 25#. I do like the sub 8oz weight though.
30-35 liters, (1830-2135 ci) would probably be the aprox size for this. This means any thing larger or smaller may still work but the benefits or lack there of would be pointed out in the tests, especially in the case of the smaller ones like MLD.
In the case of SUS, I would think that a pack that had a sub-5# base in it with 3 days of gear, (a good weekend trip), would be considered to be the largest SUS pack. Anything under this would be hard to obtain, unless you are making a lot of your own gear, have good experience with it and have a lighter than 5# load.
As far as size goes, I can easily fit a 3 day load and sub-5# base in an 1800ci pack. Anything below this size is pushing it.
I can go on all day, but I'm more interested in what other peoples comments are to this.