So, I don't know if the power goes with how many tails they have, but I guess Kurama was right about being the most powerfull Biju... I mean he countered 5 TBB with one... That's something...
Still, it made the same impact, so I guess it will... Also, Kishi won't kill the protagonist And, since it's a new "mode", Kishi wants to show it's kick@ss, so it will counter it, 99% sure...
it said that in the beginning of the manga. But the only problem is that his limits haven't been reached. He just keeps getting stronger and stronger without reason.
At first I thought that the reaper death seal would take away all of his yin power making his yang power unstable and his over all chakara 50% less. Then I thought that Naruto taking the last of his power would have been a unstable KM. (with alot of problems of its own in addition to the absorbing naruto's chakara and possibally give a reason to why the KM is acidic)
Only then when the fox's yin chakara returned and when Kurama agreed to help would he unleash the full power of the fox and possibly do something really crazy like combine SM and KM and take on the 5 other beasts.