The only time he ever used the kyuubi's power and sm at the same time was against Pein. He's never used them simultaneously again. It wouldn't do much anyway considering both modes do the exact same thing.
wouldnt do much? jiraiya in sm had real trouble controlling his chakra after being stabbed by one of the rods, from far away. Naruto, face to face with nagato, in km+sm not only resisted nagato's chakra but turned it against him, a feat that left both nagato and konan stunned.
take chakra sensing, malice sensing, and sm's threat perception and add them together. which gives you a very early warning of opponents and attacks, people's intentions etc. ie it will be insanely hard to hit, naruto in sm has never even gotten hit let alone in both modes.
naruto in sm was able to tank huge falls without being hurt. naruto in km was able to tank raikage's blows without physical damage, although he commented on their strength. put them together.
naruto's sm frog taka, combined with chakra arms. frog taka was able to one shot two pains, one of those pains had survived a raikiri hound and a raikiri in quick succession without dying, so you should get a picture of how strong he is in sm alone.
naruto's ability to merge with ma and pa if he's tamed the kyuubi, leading him to sustain the mode indefinitely, as kyuubi chakra is nearly limitless.
more powerful chakra, sm along is already much more powerful than normal chakra and leaves the user refreshed instead of fatigued after using (stated by gramp sage). add some of the most powerful chakra in the series to that.
Naruto combining the two modes would have the Sage's body, plain and simple.
