I have a primaloft Cocoon brand "over bag" . It's 2 pounds. It keeps me warm by itself, wearing long underwear to 45 degrees. The loft, or thickness of the said Cocoon overbag is not stated, but I suspect it to be about 3/4". It is certainly good for at least the additional temperature rating, when used with a sleeping bag. It does add to a bag. It's compressed volume is acceptable as opposed to down. But, for a ratio of weight to temperature, I don't think just using Primaloft is any advantage over down. Don't get me wrong, I use Primloft in other cold items, like mittens for very cold cycling, but, because of the weight, Primaloft for me doesn't add up for cold weather quilts, and underquilts. But, that's just my opinion, and experience with Primaloft.
Never more than one man left behind, so far !