I dont recall us really getting to see the development of their friendship because it has nothing to do with the main plot of the story...tbh i could care less
Kabuto has yet to except his true self and is still fighting itachi/sasuke in his mind....His only way out of izanami is for him to except his true self.
Naruto may have learned rasengan faster however he has less mastery of it. he has made many variants of rasengan but cannot make it with one hand and his rasengans size and compression is inferior to minatos