Most of the points have already been discussed here, but I'll briefly touch on the points that have been skimmed over or left out.
A "Superpower Korea" needs a PoD long before 1000, not to mention that it almost certainly requires a northern dynasty to fully absorb the southern one(s), politically speaking. While Goryeo and/or Joseon might have made some significant incursions further north, given what occurred IOTL, the population would not have increased significantly given the turmoil which occurred in the region. A divided China could potentially mean a larger Korea which expands into Liaodong, but not a significantly "larger" population, and I already stated in the other thread that China's situation will be stabilized among 1-5 states, most likely one or two, given the geopolitical situation. This scenario also does nothing to sever trading links among the states in East/Southeast Asia, as each would find it profitable to trade with their neighbors as much as possible.
In addition, given that Hideyoshi even thought about conquering China, he would most likely not have been deterred from attempting to conquer a significantly larger Korea, regardless of its size. The assumption stated above also completely ignores the fact that both Korea and Japan had plenty of trade links with each other, along with other states, both before and after the invasion. It would have been profitable for both sides to trade with other states as much as possible, so it really wouldn't make much sense for Japan to attempt to obtain resources through further explorations and/or conquest if they had plenty to begin with though trade.
On another note, the Wokou were devastating enough for China and Korea, in which the first was forced to pool a massive amount of resources in order to prevent further losses, and was one of the reasons for Goryeo's collapse. Considering that a large amount of ships were built in order to patrol the entire stretch of the coastline, along with bans imposed on maritime trade, and the fact that some Chinese peasants decided to raid Chinese coasts as well, suggests that the Chinese were not really in a strong position to support expeditions abroad from around 1350-1400, 1550-1600, and later during the Qing dynasty as well.