@Chuckmyster I have a Amplify down light as a fastpacking sleeping bag and have used it extensively in the Drakensberg paired with a bivy bag for light and fast missions. I have used it down to just above 0Deg and had a decent nights sleep, I am however a warm sleeper with a very fast metabolism. That being said it will not work for most people in the same conditions and therefore I would advise mot using it in less than 10Deg.
As a rule of thumb do not use any sleeping bag past its lower limit. The extreme rating is exactly what the names states, you will end up with hypothermia at some stage while using it at these temperatures.
As far as I’m aware neither First Ascent nor Nature hike has EN rated sleeping bags(General standard for sleeping bag warmth ratings used by all the high end brands), therefore take the temperature rating with a grain of salt.
For winter sleeping bags a cowl is a must, no point in warming up the air only to have it escape. The difference between down and synthetic is mainly a weight/space to temperature ratio. Therefore if you are looking for a light warm sleeping bag, down is the way to go in most cases. However down sleeping bags will be more expensive. Synthetic sleeping bags has the added benefit of not loosing its insulting capabilities when its wet as with most down sleeping bags, and is a lot easier to clean.
Also using a sleeping bag correctly is a bit counter-intuitive. Sleeping with everything you own will leave you colder as the sleeping bag uses your body heat to keep you warm, therefore if all the body heat is trapped beneath your clothing layers it cant keep you warm. Most sleeping bags are tested with only a layer of thermals.
For winter hiking in the Drakensberg I have Mountain Hardwear Ghost Whisperer which works wonders. The first ascent Icebreaker and First Ascent Amplify Down 900 will be perfect for the winter in the berg. If you want something light (1.2kg and below) and warm, look at Brands like North Face , Mountain Hardwear , Mountain Equipment, Marmot ect.
If you want something truly lightweight (900g) and warm, the Mountain Hardwear Phantom Alpine is on my list to Santa…Just swipe and don’t look at the price
Don’t underestimate the Drakenberg, it might not be a true alpine enviroment with snow all year round. But it can go sideways quickly.