If Mikael knew music theory

he's a good song writer but far from genius, watershed being a perfect example that he's not
If Mikael ever went to any music theory classes, he'd probably end up saying, "I knew that. I just didn't know how to say it."

This is very true in many cases. That was f ex my reaction when starting to learn theory at the end of high school. But as Iki-ilkkuja, me and several others stated in this thread; theory can open new doors. Mikael doesn't need theory, he's a superb songwriter; but it wouldn't affect him in a negative way if he chose to learn more of it.
Music theory isn´t a set of rules out there to "discover" or learn. It is created by talented musicians in the creative process.

Today Mikael would probably not be harmed by some theoretical knowledge, but it would have been bad earlier cause he would have damaged his nice ear for the opeth sound/harmonies etc.