Good questions which are more related to one another than one might think.
First up: Down suit. I have spent significant time in a down suit only once, and found it too hot for me. But I was on a lower (7,950m) peak in the Karakoram (summertime climbing season) and due to low altitude and summer-time it simply isn’t as cold there as it is in the Nepal or Tibetan Himalaya in pre/post-monsoon seasons. All times I’ve been near or above 8,000 meters I’ve been warm enough (never any frostbite) with my Patagonia DAS Parka and DS Pants serving as insulation. The exception is once on Makalu in autumn 2008. But I believe that was due to the constant wind more than especially low temperatures.
However, staying warm is only partially about dressing warm, it is also about fitness and fuel. Whenever I was high I was typically moving fairly quickly because I was well trained and very fat adapted. People who are highly sugar-adapted (this would mean most mountaineers I’ve seen, basically everyone who who hasn’t trained with a coach and/or didn’t come from an organized endurance sports background) becomes cold as soon as they run out of sugar-fuel.
So, since the key to staying warm is only partially what you wear, and to a very large part determined by what you eat, then start by reading,.
if you haven’t already, this this:
Here’s the simplified version, because the details can get complicated: The best way to achieve a fat adapted state is to spend months and months training, and incorporating fasted training into your low-intensity workouts, just one or two times per week at first, and then more and more until you’re doing all of your Aerobic Capacity workouts without ingesting calories of any kind at least four hours prior to your workouts.
Then, when you’re high on the peak, and especially on summit day, you eat what you can and (more or less) as much as you can. What foods and nutrients you consume up high is not determined by a nutrition strategy at extreme altitudes, but by a tolerance strategy. Nausea is almost a given at 8,000 meters, so ask yourself: What can you get down, and keep down?
Sugary tea? Hard candies? GU gels? Cake frosting? The only guideline is that due to the lack of available oxygen your gut won’t be able to do any real digestion, so stay away from fatty or protein-laden foods. Even nuts and nut-butters will be too much for most guts. Stick to simple foods.
My personal favorites? Barely cooked (still slightly crunchy) ramen noodles mixed with an instant chicken soup packet are one of my favorites, even for breakfast. After that I stick with GU gels because I like them and my stomach never has a problem with them (it surely helps if you drink a minimum 100-150ml of water with each packet.) And regular milk chocolate. Any kind, as long as it didn’t have kerosene spilled on it (that’s another story), also taste great and goes down easy.
One more important note, is what to eat in base camp. In short: A lot! First off, I make sure I eat 3-500 calories within 30 minutes of returning to base camp. I personally use the GU Energy Recovery shake. Then during the rest of the day I ingest about 20 grams of whey protein in a bowl of porridge or other food. Plus I take BCAA/leucine supplements, again I use the GU Energy product. These additional supplements and additions to your normal diet will help make sure your recover. The rest of the time in base camp, just eat/drink a lot. I take favorite drink mixes, hot chocolates, black licorice candy, chocolate, dried fruits, and other foods I know I love to BC so I can eat more or less all the time. This keeps your glycogen stores topped up and you’ll be as fueled and as ready, nutritionally, as you can be when you head up again.
I hope that is useful. Happy hunting!