'Viable concept, I guess, but I don't see much use for it when the 10mm is around. With .400 Corbon, you don't get a reduced size frame (if that is what you are looking for), but you do get reduced ballistic performance, reduced capacity, and much less common ammo than you do with 10mm."
I think the biggest advantage the .400 Cor-Bon has over the 10mm is that all you have to do is buy a new bbl and you have a new caliber in your stock 1911. There is no way to get .400 Cor-Bon velocities out of a .45acp but with a bbl and a box of shells, you have almost a 10mm plus you still have your .45acp when you need it.
Now the question remains, what is it good for? I wouldn't use it for self defense if I had .45acp at the ready. I wouldn't use it for target practice because it is too expensive. I guess you could use it for hunting but a .357 mag revolver is probably going to be way more accurate and deliver enough power. So, what is it good for?