800 fill simply means that it is using a higher quality down than the 700. Essentially, there is more down and less feather (the stuff with the quill). Down keeps you warm, not the feathers, since the down has the loft or the "poof" which traps the warm air. The fill power is determined by placing one ounce of down in a graduated cylinder and seeing what volume is occupied by it. The higher the fill powder, the more volume. This means that it is more efficient and can do the same job by using less down.

Hi Kate - This is a common misconception with down jackets. The 800 fill rating does not mean it will be warmer than a 700 fill jacket - it simply tells you the grade of the down that goes into the jacket, not how much of it goes into the jacket. In the case of the Nuptse jacket, it probably has at least twice as much down fill than the Thunder jacket, so the difference between 700 fill and 800 fill won't outweigh the total amount of insulation going into the jacket. The Thunder jacket is made more for milder temperatures and also to be used as a layering piece, where the Nuptse is your middle of the winter super insulated down jacket. Hope this helps

Yes 800 fill means it will be warm!