The stiffness of a frame is determined by the diameter and length of the tubes. It has little to do with the type of material.
The weight is determined by the wall thickness, which in turn is detrmined by the strength of the material. A super-steel, like 853, can be made strong enough to prevent crush damage, with quite thin walls. A standard steel needs thicker walls.
Thorn's main concern with 853 is that the thin walls are not so resistant to rust and impact damage. It is quite strong enough to use in a touring bike, but expedition bikes get all kinds of extra abuse.
I think they are being a little paranoid, and over-building their expedition models, but better safe than sorry.
Reynoldsusa.com have loads of info on different steels.