Reason a CZ is a worse gun is because its chamber is much thinner. Less metal = less strength. Most 7.62x25 aficionados that I've talked with seem agree that the TT, with its much beefier construction, can much better handle the pressure then the CZ.
I have heard of CZ kb!s with x25, I've heard of people trying to KB TT's which often resulted in failure. Even after doing dumb things like intentionally putting a 9mm in a x25 chamber. The latter was done, intentionally, by a member of this board whose line of work is destructive testing of industrial products. He did rather strange things such as ream out the chamber, put a .38 super, hide behind a berm, and pull the trigger with a cord. Result was TT-1 .38 super-0.
As for this "myth" of 7.62x25 sub-machine gun ammo. Its just that, a myth. The Com-block never loaded any x25 different. Some may have been loaded hotter, but none was specially loaded for SMGs.
I'd even wager that if TT SMG-only ammo existed it would actually be cooler then pistol ammo, because it would be loaded with a slower powder to better take advantage of the longer SMG barrel, thus you'd get a massive flash behind a slower bullet, equal or lower pressures, due to premature uncorking of the barrel.