I've found the 9mm carbine to be pretty fun to shoot (not as fun as full-auto SMGs, but what can you do?), relatively cheap to shoot, and very good "gateway drug" kind of weapons for getting new shooters who may be reluctant about long guns to put some rounds through something besides a pistol. (I seem to be some sort of magnet for girlfriends who like shooting handguns but consider rifles/carbines/etc to be intimidating or otherwise non-fun . . . my Cx4 has cured this misperception a couple times.)
The 7.62x25 conversion would be cool, but be a complete PITA to make happen. Anything that had the magwell running through the pistol grip would be a non-starter, even if chambered for 45ACP, I think, due to the long OAL of the 7.62x25 round. Even something feeding from a seperate magwell would be tricky, as you'd probably have to cut on the receiver, find magazines that would work, etc.
Might be easier to try and sort out something like a 9x23 conversion for a carbine chambered in 45 ACP.