the atom lt has fleece side panels ...
it removes some insulation for it ...
also the thermawrap pro is 80gm vs 60 gm .... even assuming the dead bird is 10% within PL1 ... and that montbell hasnt updated their insulation the last few years .... the montbell would be theoretically "warmer"
now compare it against something in a similar overall weight like the atom SV ... and it might be different
dont get me wrong ... other things matter, especially fit ... snug means warmer
but then i dont see BPLers applying the same standard for down jackets ... where they obsess over the down fill power AND the amount of fill, and the baffling, etc ....
buy synth cheap IMO ... cause unless its one of the known insulation types, its a box of chocolates, no one really knows how well it insulates ... and youll wear it out regardless
it aint something to pass down to your kids