I've seen those designs but never looked too deep into them. Maybe I've seen too many of the 'collect water from thin air' devices and become cynical. If their 90% efficiency is counting manufacturing and power costs then it could be worthwhile in cities and other areas with high atmospheric carbon levels. Same problem as the trees though, if we leave them out or use them for something they'll break down and be back in the carbon cycle. However we collect the carbon we need to lock it in place.
I would guess that 90% does not include R&D and is only about the general production costs like from manufacturing and building. So with scaling up (if they don't do R&D but just add a plant) it should be 90%.
It's a good question though, perhaps worth asking them.
