i have run in -15* in the Altras (and a few others), and hiked/snowshoed steep hills/mtns around there, but that is way too cold if I am just hiking flat/rolling terrain at a moderate pace. for this, 0* is probably pushing the limits in my lightweight hiking/trail runners, but doable. i do bring lightweight overboots with me when it gets this cold (some custom down ones that i can use moving or stationary…my winter savior), so could go lower, but i have warmer boots and do not want to risk frostbite.
i do prefer the Altra Lone Peaks for all but really technical stuff (no or poor rock plate, looser fit) and snowshoeing (i only use showshoes on steep terrain, and need a stiffer sole). La sportiva or Salewa is my go to for that type of hiking/climbing. Flat/rolling, Altras are great. And work with spikes (i love STABILicers…made in Maine, lots of models/applications).
Getting the most out of my Altras (or any shoe) has been trial and cold, cold error. A few things to maximize warmth: mid height is warmer…obviously; find some insulated insoles or footbed liners, and make sure you try your shoes on with them (big help); put a piece of foil under your insole, reflective side up (more noticeable cycling, but still beneficial with hiking shoes); try thicker wool/fleece socks (whatever fits, can boost warmth a good bit); and lightweight overboots (can be very warm depending on material). You can also try chemical warmers…hothands makes both toe and footbed warmers that are quite warm. I do not like to depend on them regularly, but I have used them the very few times I know I am exceeding the limits of my setup. And I almost always carry a pair for feet and hands in my emergency kit.
Other things you can do: start with warm feet; keep your core warm; keep your legs warm and protected from wind, especially shins (essential while cycling below in cold weather, some benefit hiking); if you plan on a long stop, bring chemical warmers, down booties, etc… to keep warm; stay hydrated; stay fueled (especially fatty foods, for me at least).
These things, in various combinations, have kept me warm in fairly cold temps. Like I said, I rarely push my hikers too low, but my cycling shoes (standard vented, uninsulated), snowshoe boots, summer weight mountaineering boots (Salewa Raven 2), and even my rock climbing shoes have been comfortable much colder than intended by boosting their warmth.
And I never ‘size up’ to just wear a thicker sock. When trying on any shoe, I wear whatever sock I plan to use with them (usually mid-weight wool), and adjust size based on activity (e.g. extra length for steep descents and kicking into ice/snow or narrower for climbing/scrambling). This is especially true with my Altras, which would get way too sloppy in the toe box and heel if I sized up. If I cannot get the warmth I need out of a lightweight shoe, even after boosting it with whatever tricks I can find, I would much rather get a different shoe/boot that fits well.
Last thing, a little off topic–in your original post, you asked about winter camping. I usually bring camp shoes for a few reasons (if I have time to wear them in camp): comfort; foot rest/recovery in a wider shoe; cleaning/drying my hikers; warmer shoe when stationary; ability to wear warmer socks. I use Salewa capsico insulated shoes. They are WP, primaloft insulated, slip on with a collapsible heel and laces, wide fitting, and decent tread for snow and ice. I have the low version, but them also come in a boot. If I don’t have time to walk around in them, I just stick with my down booties.
