Here is a link to HG's site on the right you will see the table breakdown for the incubator.
A winter incubator not only has more down but it also has larger baffles, the larger baffles allow for maximum loft which is in excess of 4". The baffle height for the winter version is 4" but the outer shell is differentially cut, so this usually gives an extra inch (or more) so there isn't compression when used.
By just adding more down to effectively change the rating has a upside and a downside. Depending on how much extra is added you may not be able to get full loft because of the compression when used. Most times adding a ounce or two doesn't hurt the loft (as long as there isn't 30% or more overfill already in the quilt), but adding more (think 4+) may do more harm than good if the baffle height isn't adjusted for properly. By adding a ounce or two has a tendency to help those whom sleep on the cold side to get closer to the rated temperature, but not in all cases.
Hope this helps???????
Formerly known as "Cranky Bear"....
"yesterday is history, tomorrow is a mystery, but today is a gift---thats why its called a present" - Master Oogway
It's always best if your an early riser!
I like hiking as it's like exercise!