There is a common misconception that Naruto and Bee are immune to genjutu. That is not true. As we saw with Bee, Genjustu can be placed on him, but within a few moments his tailed beast could snap him out. This means that if Sasuke would use genjutsu on Naruto and follow through with an ultra fast attack like amaterasu or body flicker technique, it would be effective. (Obviously I'm not saying that Sasuke can beat BM Naruto that easily, it's just a possible attack combo he could use.)