No possible power up can put Sai on Sasuke's level. Moreover, if Sai tried to attack Sasuke, Sasuke's response would probably be, "You don't interest me at the moment."
Moreover, while Sasuke is still a dark character, why would he betray the Alliance at this point if he wants to be Hokage? It negate the character development since he meet Itachi. I think Sasuke has not cause to betray the Alliance, and the only reason Sai would attack is if Sasuke does betray the Alliance.