For wintercamping I have good experience with a total package;
Ofcourse a good sleeping bag (-30degrees C) and a good inflatable pad; a thermarest normal length.
But to fill the gap between really cold and acceptable cold, I buy a sort of thin foam and use it as a 'carpet' covering the complete floor. The foam is about 1mm thick and has less weight then a normal closed cell pad.
The advantage is that when you roll over during the night it's not that cold. Also in the morning when you have to pack your stuff again, it's not that cold.
I buy the stuf in an outdoor shop and it is not that expensive. Because it is very thin, it doesn't last that long. normally I use one piece for one week.