Saying, "You are so busted," only equates to being "very busted," because of the emphasis we place on so.
"You are so busted!"
Is the kind of sentence that a child would say to their sibling when they're gleeful about the potential trouble that their sibling will be in. The emphasis and saying "so," instead of, "very" adds another meaning to the sentence.
"You are going to be in trouble and I am excited about it!"
This illustrates my point that so=very is incorrect, and by that reasoning, "I can only do so much," can be read as having an ulterior motive as well.
The kind of situation where someone would say, "I can only do so much," is usually in an apologetic context
"The air conditioner is broken, but look, I can only do so much."
or as a dismissal, where the speaker is not actually that interested in the problem or issue.
"The air conditioner is broken, but look, I am very busy right now."
I can only do so much doesn't have a literal secondary meaning, but it's implications deliver another message then immediately appears.