To answer your question...

Steemit has 29M Steem which is equal to 57238 MVESTS.
The No. 1 witness right now has 93620 MVESTS backing him.

Steemit has not voted for witnesses yet. If Sun does it, he can shake up things a lot, but he cannot get 20 new witnesses in the top 20. (witness No. 20 has 77113 MVESTS)
This means that the top20 witnesses can fork his stake out of the chain if 17 of them choose to do this (if needed).

Only if sun somehow acquired double the stake of steemit, he could do whateverthefuck he wants.

Edit: I forgot to factor in the accounts "steem" 11.6M Steem and "misterdelegation" 19.3M Steem.
That is enough stake to replace all top20 witnesses. Whoopsie.