This is my first post, written after reading the whole thread, I believe. Here's what I can add:
About Availability: I just purchased a pair of M11s (172s) after demoing them this season. The ski shop (in Sunriver, Oregon) special ordered them from Atomic on some year-end deal and had them for me in 2 days for about half of MSRP, or the same cost that I watched these skis go for, new, routinely on E-bay (not the inflated buy-it-now prices but the no-reserve, open auctions). So I am thinking it may be worth seeing if B5s are available in a similar manner.
The Ski: I weigh in at 200 lbs and used to ski on 205 cm GS skis routinely. It's taken me a few pairs of skis (all atomics!) to get down to this length. While I understand that the M11 is supposed to be softer than the B5, for my money, the M11 does it all just fine -- they handled an ice day in January with more joy than any GS ski I have been on and, in the powder, they are amazing as well. In my mind the holy grail of ski design -- creating a true all-mountain ski that compromises nothing -- has been achieved.
The length: I didn't try the 162s, though I was tempted to after reading the earlier postings. Basically, I believe if you know how to ski these new skis -- which I am definitely still learning -- I would expect stability at high speeds to be a non-issue, with either length. Keep 'em on edge, even a little, and, as you speed up, accentuate the forward/aft differential between your feet, pushing the uphill ski forward more. At least, that's what I am working on with them and it's a heck of a lot of fun. It's sort of like creating a longer platform out of the two skis and it feels akin to what they are doing on the World Cup these days (okay, that last part is probably only in my mind).
I'm guessing that the longer ski may give a little smoother ride in crude and maybe demands a little less perfection, but I don't know. I just know that, once I skied the M11s, that I had three other pairs of pretty good skis in my basement that were instantly obsolete.
I really appreciate reading all the earlier posts. It's a pleasure to find some common ground. I hope I have contributed appropriately!