Generally the more down in a jacket (fill weight), the warmer it is going to be but this assumes the down is the same quality. However, down quality can vary significantly and is measured by its fill power. A higher fill power down will keep you warmer than a lower fill power. To make matters worse there are two widely recognised methods of measuring Fill Power - the 'US' Method and the 'EU' Method. Rab use the US method to calculate their fill power. Both methods work on the same methodology but the 'US' Method gives a slightly higher figure than the 'EU' method for the same quality down - so an 800+ Fill Power (EU Method) is approximately equivalent to an 850+ Fill Power (US Method).
Wherever we quote a Fill Power for a specific garment (or sleeping bag) we specify that the measurement is either 'EU' or 'US' method - so that you can be sure of the facts. Because the difference is small, and for simplicity, when writing in general - such as for this article - we won't make the distinction.
A general rule to follow: 650 fill power is considered to be a high quality down for outdoor gear. 750-850 fill power is considered to be in the upper bracket in terms of down quality. 900+ fill power is even higher quality again but is expensive, resulting in higher priced garments.
This is simply how much down is put into a garment or sleeping bag and can be used as a rough guide to effectiveness - e.g. a jacket or sleeping bag with 350grams of 850 Fill Power Down will be warmer than one with 350 grams of 750 Fill Power Down.
The comparison assumes that both sleeping bags are well constructed and the down has enough volume to expand into (called Lofting). Simply stuffing more down into a compartment could be counter productive if the down can't expand to its full potential, sleeping bag designers focus on how to make the compartments work best to let the down loft while keeping it distributed over the body without creating cold spots.
This is a relatively new development and is a treatment applied to down that makes it water resistant, there are a number of different treatments and names, Nikwax treated Hydrophobic Down is the one used by RAB.
Although it is not waterproof, Hydrophobic down absorbs a lot less water, dries more quickly and retains loft more effectively in damp conditions. If Hydrophobic down gets soaked it will still 'clump' like regular down and lose its insulating value.