so I guess the question is does 6 ounces of 500 fill down in a North Face equal 4.2 ounces of 800 fill down in a Montbell or Kuiu?
Yes (quality being essentially equal, and provided each is allowed to loft to equal thickness).
Higher lofting down primarily provides two things...
1). The same warmth (loft) for less weight.
2). Smaller compressed sized (ie, in a stuff sack).
Take a sleeping bag as an example... the classic mummy bag is one with a 6" loft / 15* +/- temp rating:
A Western Mountaineering Versalite (6" loft) uses 20 oz's of 800 ci down... 800 x 20 = 16,000 ci
The same bag, with 550 ci down will require 29 oz's of down. 550 x 29 = 16,000 ci.
Warmth will be the same. Just more weight and bulk with 550 ci down fill.
Finally, some make the argument that 800 ci lofting down will last longer than 550 ci down... if both are of high quality, I'm doubtful of that argument.
