minato already sealed half of kurama's chakra using the demon god seal......the equation is simple:
10 tails=sum of chakra of all the 9 bijuus.....
and we already know that the 10 tails hasn't gained the perfect or the most powerful form yet....if the half of kurama's chakra is already sealed then how can the juubi gain a perfect form?:shrug:
and a random thought...if somebody were to use a demon god seal on the 10 tails would it work?? how big a tummy does this demon god dude have?? :D