He's also strong beacause of whats driving him ( Bring back Sasuke etc.)
I mean sure use the Kyuubi's powers to cheat a little but isnt that sasuke relying on?
He uses the Seal as a shortcut but Naruto only uses the Kyuubi when there is no other way out hence hardly any control the more he progresses.
He motivation is becoming the strongest and that whats drives him to push is limits.
Even if didnt have the Kyuubi...whats driving him is his fighting spirit.
Short cuts or not he's considered strong in my eyes.