"... the snow conditions are similar – in all the videos, the skiers can find the bottom – which is really all you need, at least as far as snow depth per se is concerned, in order for an all-mtn. ski to be suitable.
Rather, it seems the difference is that these conditions would favor a shorter, more maneuverable ski, with particularly solid edge hold (since it’s quite steep and there may be ice or refrozen snow under that powder). This suggests perhaps a narrower ski as well..." (chemist in the original post)
For my 2 cents worth I'll say that short skis are not good in soft snow, period, unless you just want to chop at it (like at the start of the last video above). Regardless of the existence of a hard bottom underneath, you still need to deal with the soft on top. Fore/aft balance is critical in every kind of deep snow, and the longer the ski the more you are able to relax, which leads to better skiing in every situation, including trees. I probably seek out and enjoy funky snow more than most people, and from my experience I recommend going as long as you can. Based on what I read on this forum I think the vast majority of self-described good skiers are on skis that are one size too short for all mountain skiing, but that is just my armchair opinion. Shorter does not necessarily equal "turns easier" because it is harder to get into a turn if you are not totally balanced on your skis. Dawgcatching is making some smooth on-demand turns in the first video, and I would bet that had he done the same run on shorter skis he could have made the same turns at the same speed, but he would have expended more energy because it would have been a little harder to maintain balance.
Width is certainly relevant for soft snow, but I would always sacrifice some width in favor of length, which adds stability in deep snow of every kind. Shorter skis make it easier to recover from bad technique, but they also lead to it once you start picking up speed off piste.
My perspective is colored, perhaps overly so, from having spent over a decade using 160 cm Atomic SL skis as my only off-piste boards. I found they turned more confidently on steeper terrain in soft snow than the (again) very limited selection of new skis I demoed last year (172 cm Line Prophet 98s and 170 cm Volkl Kendos), which gave me the confidence to take them pretty much anywhere -- they were great fun skiing off-piste in Europe. By contrast the Prophets, in particular I found ponderous -- I don't think I'd feel comfortable taking them into a steep, narrow chute. But my Atomics have some definite downsides. First and foremost, when I took the Prophets onto chopped-up heavy crud on more open terrain, it really was a blast to make fast, sweeping GS turns with them -- something that, as you say, would not be relaxing at all on my Atomic SLs. I'm looking forward to seeing how my 168 cm MX88s and 170 cm Laser SCs behave.
Edited by chemist - 10/6/13 at 5:52pm