Fill power is a measure of how many cubic inches an ounce
of goose down takes up under standard laboratory testing conditions. A
high fill-power down is thus warmer for its weight than a lower
fill-power down. So this jacket, which features 1.6 ounces of down, should be just as warm as an equivalent 800-fill jacket with 2 ounces of down, for a savings of 0.4 ounce. (Or since we're in gram-counting territory here, let's use the larger grams-saved number: 11.3 grams.)
To add to this grams savings, the jacket also uses a ridiculously thin 7-denier nylon thread, which means that if you laid out 9 kilometers (~5.6 miles) of this thread, gathered it up, and put it on a scale, it would weigh all of 7 grams (0.25 ounce). This is also pushing the limits of what's currently available on the market. (As with most ultralight fabrics, you're making a trade-off here between weight and durability/abrasion resistance.)
Finally, the jacket does away with anything that might add extra grams. Its only "accessories" are a front zipper and elastic cuffs on the wrists. No pockets, drawstrings, Velcro closures, or anything else.
The total weight of the jacket (size medium) comes in at 4.8 ounces. As with all things ultralight, you pay more to get less, at least in terms of weight. And shaving down those final few grams often bumps up the price significantly. The Plasma 1000 Jacket retails for $269, or $56 per ounce. (If you stick to grams instead—$2/gram—perhaps you can better rationalize it.)
As for me, I'm always intrigued by the latest efforts to create the lightest-ever gear and I've always been impressed by the ultralight offerings from MontBell. And though the Plasma 1000 is interesting from a grams-saving perspective, the jacket I've got on my covet list is the MontBell U.L. Down Parka, which offers a lot more warmth for less price ($175, 9.0 ounces) and for less additional weight than another 10 tablespoons of water.