All of the jackets listed are great, but there are some very different insulation levels listed. They are all baffled, all have good quality down (850, 900 fill power), the question is how much of that down is there. In a given jacket, more down=warmer. No way around that.
Of the original 3, the feathered friends jacket will be the warmest by a significant margin. Over 13oz of 900fp down fill, compared to 8 or 9oz in the others. That’s 50% more insulation than either of the other jackets. The kuiu in particular has a stretch wpb shell so it’ll be heavier and bulkier for the warmth.
The montbell is a great jacket, but its only got like 6 or 7 oz of 800fp down fill based on the jacket weight, I could not find a fill weight. Frankly I would not buy a heavy down jacket sight unseen if they dont publish the actual weight of the fill. Its a very good quality jacket, but its not in the same “heavy jacket” category of warmth of some of the others.
Rab positron pro is about 10oz of 800fp down fill and would be a decent option at much less $. Rab makes an 8000m down jacket with 19oz of fill, thats warm. The other rab jackets are mid or light weight jackets with less insulation, 6-8oz of fill.