the salary is not just related to performance. it is "markable value"
Taiwan's market is very small. so Wang's salary will be pretty much reflect what's his ability to win. On the other hand, Japan is a much bigger market. MLB can get more money if they have a Japanese player on the team.
Why Yao Ming can get 15M plus each year? a lot of the reason is the Chinese market. I always think if Wang want to get more money, his agent can't market him as Taiwanese player. If he can be marketed as a Asian Chinese player, 10M plus salary, no problem.