The China above is never the China we'll see in game. We'll never see the China that in the 16th century could muster hundreds of thousands of men to march against Hideyoshi in Korea (okay, ruining itself in the process but hey...)..
China was able to put about a quarter million soldiers in Korea through a mostly land route throughout the course of the entire war. However they never achieved battlefield strength above 80 thousand troops, if that. This was a war fought around 1600 the same time frame that the Spanish Armada was sailing to it's doom. The Spanish Armada was part of an invasion force of 50 thousand sailors and soldiers. China had at this time a population of around 150 million, Spain had a population of around 8 million. Spain concurrently with the Spanish Armada was fighting on other fronts worldwide. After the disaster of the Spanish Armada, Spain was able to bounce back and overwhelm the English retaliatory invasions and show itself to still be a major naval power, albeit clearly not the overwhelming force it had previously seemed. China faced a relatively easy campaign in Korea given the amazing effectiveness of the Korean navy after the command shakeup but ended up indebted and losing internal control due to the strain of supporting the expedition.
The point I'm getting at here is that China's involvement in the Japanese invasion of Korea doesn't highlight the power that China projected, it shows us just how little China was able to project beyond it's borders. This should not be surprising because China had such immense difficulty projecting within it's borders as well, it's not easy to rule a country with so many powerful internal forces around.
I would agree strongly with the thesis that China is way to easy to invade by a European. Such an invasion would be all but unthinkable until the last few years of the game. But I'd also object to the idea as China as some sort of population and scientific juggernaut. China was a giant on feet of clay and ended up fighting way below it's weight. The latter reason is why China comes across as too weak. The game engine isn't set up to portray a situation like China with it's vast internal power but weak projection. The result is that the difference is split and China is too strong outside its borders but too easy to invade. So if you think that China needs a buff you are wrong. They don't need a buff, they'd need an overhaul to reflect their internal situation.