Wrap your minds around this, if you can.
Easily the strongest base technique in Naruto's arsenal, the Rasengan, in my opinion, has PLENTY room for upgrades. We've already seen the regular Rasengan, the Oodama Rasengan, the Wind-element Rasengan, the Kyuubi Chakra-infused Rasengan,, and the Rasenshuriken. Now, remind yourselves of the damage each one inflicted (except the Kyuubi one, which kinda missed). The normal Rasengan was enough to send Kabuto into a boulder AND cause internal injuries, even WITH Kabuto's advanced Healing Ninjutsu. The Oodama Rasengan killed an Itachi clone in a SINGLE BLOW. The Wind Rasengan is like a spinning ball of razorblades, so it's kinda hard not to see the damage potential. Now, the Rasenshuriken is easily Naruto's strongest Jutsu(so far), capable of totally, utterly, and completely destroying a Shinobi's body(Kakuzu only survived due to an extra heart, which was on the verge of failing after being hit). Now, imagine if you will, Naruto activating the Kyuubi's Chakra, Sage Mode, AND the Rasenshuriken. The result? One large, swirling ball of pwn.