While I'm with you on that it would require pretty much a complete redesign
S2Ks are blowback and you would have to change the action to something other than that to handle 30C and that would also complicate the way they fold. Plus a 30C capable magazine would be too long to fit in the grip as is.
Doesn't mean you couldn't make a S2K like gun in 30C though it would require pretty much a new from the ground up design
Oh I know. It wouldn't really be feasible. A full blowback version would be harsh and a lot heavier, taking away much of the charm of the Sub2K.
A redesign to a locked action would likely do much the same thing -- take away the charm (and likely add a lot to the price).
That's why I mentioned the shortened .30 Carbine round I remember from the 1960s or so. If I'm estimating the ballistics correctly for it, you'd have a 110 grain bullet moving at around 1,300 fps.
That would give around 400-450 foot pounds at the muzzle and should be just inside blockback action territory.
Of course, that's not much more energy that some of the 9mm loads available today, but it could give you more ammo in a more compact magazine...
The interesting question, though, would be the magazine/grip. Would it allow a comfortable grip while still having the mag through the grip?
It would be a bit deeper front to back, so it might not be nearly as comfortable as the Sub2Ks chambered for pistol cartridges.