Ok, it's pretty obvious what power Itachi gave Naruto

[QwertyHero]

through practice and hard work.
when he tried the first time he missed because of course he missed
it was his first try.

Yea, but if he had so much potential, he would have at least got close, he totally missed.
I'm sure Itachi didn't miss like that his first time..

 

[Xero]

I believe Itachi gave Naruto Susanoo's super sealing sword.

 

[Achilles]

yaaa i thnk its a power to put off amaratsu or it may b a new type of sharingan!!!!