nidaime and i come up with this thread.
I do believe Minato mastered the Sage Arts. I find it hard to believe Jiraiya would have an asset like that and not train his prized pupil in it or not allow his prized pupil to go and get training by the toads.
So you ask "why not Naruto as well?" well Naruto already has the fox which Jiraiya felt was Naruto's answer to everyone of his skill problems UNTIL he saw that the fox chakra caused physical damage which is probably why the training stopped.
Here is the proof I see:
Minato was known for his ungodly speed.. Senjutsu is known for increasing physical attributes like speed and strength. No one so far from what we have seen has been able to match or surpass Minato's speed by normal means. Senjutsu would explain the insane speed.
Flying Thunder God is a modified version of reverse summoning and Minato had to have gotten the idea from the Great Sages. We both know that Minato used a tag and a seal to start the FTG. Naruto notes that Fukasaku makes a seal to initiate the bolder lift. The Sage Arts are linked to FTG.
Death God Seal vs Kyuubi.
From the Sarutobi vs Oro fight we saw that the user of the seal had to physically hold the subject and forcefully rip their soul out using their own spiritual/chakra strength. We saw that Oro was able to resist and pull back due to being stronger than Sarutobi at the time... So explain to me how Minato could have over powered the 9 tails Demon Fox without some MASSIVE boost in power?
I can see Gama holding the Kyuubi for Minato but I doubt Minato had enough spritual/chakra strength to overpower the Kyuubi, rip out its chakra essence, split it in two, seal half in the Shiki Fuujin seal and the other half in his kid without some sort of power boost...
The explanation: SENJUTSU
I always questioned how he was able to do it but now I can see the answer. Minato took Senjutsu and built a fighting style around it and developed his own techniques with it which would also explain why no one can mimick them except for the people he taught them to personally.