no untill sasuke shows his true power why people keep saying powerup
Because he's showed us what he can do. Contrary to popular believe, EMS only takes away the side effects of MS. It does not grant the user to unknown powers. So yes, what Sasuke needs is a power up, or some form of higher power than he currently has.
Naruto, for example, did not show his "true potential" when he went Bijuu Mode. He earned a power up by teaming up with Kurama.
On topic: The show is called Naruto. (Spoilers!) Sasuke cannot beat Naruto in the final battle. At most, Naruto and Sasuke will tie and will both die.