The First Ascent Down Light is a good bag, but not at all suited for cold conditions. I’ve slept in one, wearing full thermals, when the temperature inside the tent was 10°C and I was waking up every 30 minutes from shivering - you will have a VERY uncomfortable night if it drops to 5°C. Don’t skimp on your sleeping bag - shave weight on everything else, but make sure you can stay warm when the temperature drops.
First Ascent Ice Breaker is a suitable sleeping bag for most South African conditions. Also check out Naturehike’s sleeping bags. Always remember to gauge the sleeping bag on the comfort rating. Don’t plan around the “transition” or “extreme” rating - you won’t die, but you definitely won’t have a good nights sleep. Warmth and weight go hand in hand - no special magic there.