The Enterprise is the top ship of the fleet in this period of time.
A villian can feasibly be rolling around in superior ship (perhaps a prototype), but not outrageously uber and unstoppable as that would make no sense. And I dont think the ship itself would be the weapon that changes the game.
I don't like the idea of non-scientific rogues suddently acquiring special knowledge and the technology to conceive ships or weapons (Shinzon and his Schimitar is very guility of this one) that vastly superior to those made by the actual millitary divisions of the Federation or the Kingon/Romulan empire.
This again makes me wonder how Johnny H managed to acquire weapons/abilities of such game-changing power that never existed in the original time line.