I must say that even though there appears at first sight that the baffle construction is better in diagonal construction, it is not enough to compensate for the weight penalty. Using box walls and a bit more down seems to come out lighter.

https://www.phdesigns.co.uk/minimus-down-sleeping-bag-k-series

This also is an example of the cost of good down,this being a prohibitivley rare class, as everyone could probably not be furnished with this type, theres just not enough to go round. The price difference in the different teperature ranges of this model is extrodinary.