That may be a bit enlightening. With a longer barrel, you do pick up additional energy from the extra velocity, but the 357Sig doesn't get any more benefit from being fired in a carbine than the standard service calibers. The energy graph goes a bit higher than others simply because it is a higher-velocity pistol round and velocity is a key figure in energy. If you created a momentum graph, it would probably be about the same as a 40 S&W.
I think there is probably nothing wrong with your suggestion, but I don't believe that it would really make that much of a difference either way. A full-auto burst of 147gr. or 124gr. 9mm bullets is going to be just as damaging from a practical standpoint as the same burst with 357 Sig... it would just cost 50% less. Unless a LEA is going to purchase sidearms in 357 Sig, which I think is unlikely most places, it doesn't make sense to have carbines and subguns that use the cartridge.