My experience is that the warmth these liners added is exaggerated... and the salesman has got to be smoking something. The warmest liner I saw on the Sea to Summit website claims is 20F. Thermolite isn't magic. It provides less warmth per / oz than down, primaloft, Climashield or polarguard... I believe around the same the high loft fleece.

The lightest option would be to use a down bag or quilt that appropriately warm... but that might be cost probative. If you have a bit of extra room inside your bag then you can use either insulating clothing or a small quilt inside the bag to boast the warmth.

I use a quilt which works well to 35F when wearing a light fleece hat and summer weight base layer. When I add a down baklava, gloves, a mid-weight base, and a ultralight down vest I am good to 20F, or if I am using my warmest down jacket and insulating pants I am good to around 0F.

In the dead of winter I switch from my quilt to a down bag and my daughter uses my quilt inside her sleeping bag to boast it's warmth.

As others have observed.. the mylar emergency liners are not very comfortable but can add a surprising amount of warmth (especially when is very cold) due to functioning as a vapor barrier. If you are looking at <10F conditions a vapor liner is a good idea to preserve the insulation of your primary insulation, but I would want a material that is a bit more comfortable than mylar next to my skin... and for vapor barriers to function properly they need to be very close to your skin.

I have a a few notes and links to other information sources on my sleep systems page and on my recommendations for insulated clothing