When Minato used the Reaper Death Seal on Kurama he sealed half of Kurama's Chakra in Naruto and the other half in himself. Naruto has always had only half of Kurama's power since the beginning, if he did have the other half I can't imagine how hard it would have been to control it back when it went wild regularly.
Seems Madara has known Obito's weakness from the beginning, which explains why he has been stalling Hashirama, since Hashi has the most powerful Sage mode of the bunch. Orochimaru does know sage mode, even if he didn't know it before, he does now that he obtained his power from Kabuto.
I'm guessing that Tsunade also has a Sage mode, perhaps that seal on the forehead is made up of Natural energy, which gathers for years before being released as a more powerful sage mode. That would explain the huge boost in physical strength that both her and Sakura have after the seal is ready. If that's true then Sakura can technically go into Sage mode at any point since her seal is finished charging!
I know that when the seal is released it heals the user constantly, but what if Sakura's version of the seal is offensive instead of defensive, what if she gains massive strength powered by Natural Energy when the seal is released?