I suppose hypothetically there would be more of an upside for Romney. But I suspect John Roberts is smart enough to know that such a decision would destroy the legitimacy of his court, so I doubt he would vote for a total strikedown. The vast majority of the bill's provisions haven't got the slightest thing to do with the "mandate," so I don't know where they would derive the legal justification for that kind of decision.
If the Court does choose to strike down the entire law on the basis of the mandate, the Justices will likely point to the lack of a severability clause in the original legislation. Absent such a clause, it's up to the Court has to decide how the mandate interacts with the rest of the law, and the lower court cases on the mandate provide ample precedent for any direction they decide to go.
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