The higher the "fill power" of the down, the less is needed to achieve a given warmth. Thus, everything else being equal, a 900fp bag will be more efficient -- and ligher weight -- than a 600fp bag. What are you giving up when you buy a high fp, ultra lightweight bag? Moolah -- and lots of it!
This REI website has good info. on choosing a sleeping bag:
Finally -- an important caution! There is no standard for temperature rating. Warmth is subjective. Also, gear makers all assign their own ratings. Many are "optimistic" with their ratings -- you may freeze if you take them too literally.
Western Mountaineering, Montbell, Golite -- and REI (more or less) -- are pretty honest.