I don't think speed's the issue- Kostya could deal with speed pretty handily. Toughness, chin, and conditioning could be. I think Kostya's the better boxer, but to me it boils down to how a 140 lb Shane handles the clean shots when he gets caught and whether Tszyu can last long enough to win on points since Shane has a great chin. It's a tough call. I don't know who I'd pick to win.
Mosley would win easily, I'm not saying a complete destruction or anything, but a wide UD or late KO/TKO.