Old thread I realize, but I tried a bunch of UL jackets recently.
Montbell plasma 1000 alpine parka (with hood)
Montbell plasma 1000 jacket (hoodless, Japanese version with hand pockets)
Montbell alpine light
Ghost whisperer hoody
Ghost whisperer vest
Cumulus primelite pullover
I can say without question the greatest (commercially produced) jacket ever is the Plasma 1000 PARKA (the one with a hood and 3.4oz down). It's warm below freezing with minimal base layers and fully featured at under 9oz (full zip, hood, hood adjustments, waist cinch, zip hand pockets)
The Plasma 1000 JACKET on the other hand is a worthless waste of money that isn't even warm at 60 degrees. It has a pitiful 1.6 oz down and no waist cinch so you lose all your heat out the waist. Avoid this jacket. 1.6 oz is not enough fill to lift the baffles. A 5oz fleece like a peloton 97 or macpac nitro is a better layer. Just get the hooded parka which is worth its weight in gold.
The cumulus primelite is the best warmth: price:weight piece on the market, but it has a distinct European V cut. I am very athletic and there it's cut perfectly, but has zero extra room for an American gut. Zero. At 3oz 900fp down for $200 it's a great great piece if you are skinny or athletic. (Muscle or fat bulk will cause issues with this piece. Be warned)
I love my alpine light. It's slightly warmer than the plasma 1000 parka as it has about 10% more fill and a better baffle pattern. It's 20d material is somewhat more durable. I mostly use this on shorter trips or around town.
Last I have a MH GW vest in a size smaller than I usually wear. Since it has no arms I can get away with a medium which fits my torso like a glove. I layer this under any of the other jackets for a significant warmth boost for only 5oz. plasma 1k parka + gw vest = 5000 cuin fill for 14oz. Toasty well into the 20s and probably lower. The 1k parka + cumulus is 6000 cuin full for 15.5 oz, a true UL masterpiece combo probably good into single digits with a smart setup (i.e proper clothing over the rest of your body plus a shell as needed)
Hope that helps. Assuming you're fit, buy the Cumulus Primelite not the P1K jacket. The primelite is a much much much better/warmer/better value piece. Or just step up to the P1K Parka. It is stupid warm and well designed at 9.1oz for a size Large. Overall the 1k parka is best puffy followed by the cumulus imo. The 1k hoodless jacket is a scam. (And I say this as a happy owner of many montbell products, and strongly recommend all their stuff, minus the 1k hoodless jacket)