I don’t use liners. The main function of a liner is to keep your bag clean. They aren’t very efficient in terms of weight to warmth, are very fiddly and I generally don’t find them very comfortable when used inside a sleeping bag.
The claim of adding 15 degrees is an interesting one - adding 15 degrees to a 10c bag is very different from adding 15 degrees to a -40c bag.
As someone who hikes in the Drakensberg every winter, here’s a free tip: your sleeping bag is not something to mess around with. Winter is a lottery, it could be 0c overnight, or it could be -20c. I would not hike to the top outside of summer without something rated less than -7c. If you bag isn’t warm enough, I hope you are very close with your team mates 
On a side note, if you can find something by Sea to Summit, Mountain Hardwear, Mountain Equipment or North Face - compare the specs to our local brands and you will see why I don’t buy locally made sleeping bags. A Mountain Hardwear Hyperlamina Flame has the same warmth rating as the locally made Drakensberg-winter-rated down bags, but weighs about 400g less, is considerably cheaper, is also water resistant. Notably it is also a synthetic bag, which gives you context how far behind the rest of the world we are.
Sleeping bags are expensive, so it is worth doing your research and buying a good one now rather than ending up with a cupboard full of sleeping bags that you used to use, before you found something better.