High Sierra Sniv- 29oz, 16.5oz of down
Synmat UL- 15.2oz
Montbell UL pants- 7.8 oz, 2oz of down
Down Jacket (Assuming same model as pants)- 8.0 oz, 2oz of down
60oz sleep set up, with 20.5 oz of down
1- lightest way to add warmth is to add down
2- Nearly all down clothing (excluding perhaps Blackrock items) are less than 50% down
3- sleeping pads are often the weak points of cold weather set ups
If you were carrying the down jacket and pants anyway (as I'm assuming you might in cold weather), than speaking strictly of weight this is an efficient option.
A 1/4 inch pad from prolite gear (for example), in a large size, is cheap at 20$ and adds 7 oz at the most. you could even ditch the down pants (assuming they are only for sleeping) and come out on top.
The lightest option for a sleep set up would be to have a CCF pad and a quilt with more down, but this excludes worn items.
In conclusion, shelling out a couple of bucks and a few ounces for a CCF pad seems like the logical choice. I have a blue foam pad from BIG5 that was 6 or 7 dollars, weighs 6 ounces, and is my 4 season add on for sleeping in the snow