The thing is Naruto automaticly uses Kyubis chakra, example: Havent they always talked about Naruto having a greater chakra stamina cuz of Kyubi? Doesnt it seem very unlikely that Naruto can summon Gamabunta with his own chakra when about a week ago he only could summon tadpoles, the thing that is reasonable though is that he summoned Gamakichi with his own chakra.
The thing is that when Naruto "asks" Nine tails for power, he gets a much larger boost of power and speed etc, no?
So the logical answer would be that Naruto taps into Kyubi's chakra when he makes that 1000 clones attack or summoning Gama Bunta, thats what he learned from Jiraya, not learning how to ask nicely to a demon :S

Well there are 2 resons for me thinking that Itachi surpasses Naruto with power 1, Well yust use common sense and think about it. 2, Why would the Akatsuki send someone who is weaker than someone they are supposed to retrieve?