I was wondering: You've probably been asked this before, but Sheogorath has 10,000 Magicka per the Prima Guide Book (under the topic "Killing Sheogorath"). Since by virtue of possessing the Staff of Sheogorath, you have supposedly both the power and the authority he held over the Isles. It would still be in-character if you were able to progress to 10,000 Magicka, as long as the SI MQ was completed.
Would you consider modifying your Uncapper mod to allow for that progression, if your character is Sheogorath? Thanks for considering this request.
Reaching 10000 Magicka just by your character attributes is unlikely. And going beyond the level 200 cap for natives attributes is not good because it will reset to 0 past 255. That's why I limited the Uncapper to 200 (which leaves some space for eventual script bonus alteration your character can encounter during the game)
Also with the Uncapper, if you ever reach level 200 in a magic school, each spell in that school will cost 0 or 1 magicka to cast. So It's in kinda like relatively having a unlimited pool of magicka.
Now lore wise, if you really have your Sheogorath-promoted character to have 10000 Magicka, the best way is to simply do it by script, giving a Magicka Ability Bonus to the player once he becomes the new Sheogorath