aw cmon man. Aside from not having to stand still in order to achieve sage mode, if naruto is way stronger than he is in real life while fighting the kyuubi, then he would have fought it back in part 1. What he has shown to do in his head does not contradict what he has been shown to do with sage mode:
His sage mode physical strength was the same. He did not use over two rasenshurikens while in sage mode. If it was a straight up battle with the fox, sage naruto would have killed it, but the fox only started to gain the upperhand when he corrupted naruto with hatred when naruto tried to pull the chakra from it. And once again the ninetails and/or the third raikage are in a league of their own in speed and power when compared to sasori.