Just how powerful do you think Po from Kung Fu Panda is when he has mastery over chi?

First off: 1st movie >>> 2nd >>> 3rd > All other Kung Fu Panda shows/other stuff with the exception of some

Po has PIS and plot armor on his side. I can't believe I'm saying this but Po > Bull Kai > Oogway

This is actually a pretty good question. Kung Fu Panda was a masterpiece. The movies were part of my childhood and I really enjoyed them.
So, here we go: Po is really strong naturally. As both a panda and the Dragon Warrior, Po has an unusually strong chi/ life energy. Po is also able to manifest his chi in the form of a Chinese dragon as he did in the 3rd film. While he has never shown the ability, pandas have been shown to be able to heal wound by using chi. While there is no "Panda style" of Kung-Fu, Po's fighting style is reminiscent of Tai Chi, which emphasizes simple, flowing movements and redirection of attacks. His fighting style also resembles Zui Quan, or "drunken kung fu." He beat Tai Lung easily, who destroyed the furious 5 + Shifu. When he mastered Chi, he surpassed Master Oogway, who was the most powerful character in the series.
Po has actually really good feats of durability, aglity, strength, etc. Not to mention that he is a quick learner, like Wuxi finger hold, he learned other types of moves just by seeing them. He could beat a lot of people actually. So respect Po!