A 900-fill-power down jacket that has only 30g of down fill won't be warmer than a 450-fill-power jacket with 100g of down fill. In theory, 50g of 900-fill-power down would be the same warmth as 100g of 450-fill-power down; it would also compress to about half the space. Very few jacket makers report fill weights, though. And any simple formula for warmth calculation won't account for differences in how the down fill is distributed within baffles and around the jacket.
Your other clothing layers, the jacket's shell material and many other design details also figure into how warm you will feel. Couple that with differing metabolisms of people and widely varying weather they might encounter, and you can understand why a temperature rating on a jacket wouldn't be meaningful.
Synthetic fills have no equivalent test to down fill power, and each brand of synthetic insulation has unique characteristics. So, it's not possible to use down fill power to compare insulation efficiency to any synthetic fill, and there's no way to compare the efficiency of two different synthetic fills.
