It would be unprecedented and appealable, but mostly it just wouldn't happen. Judges aren't required to adhere to the guidelines (anymore), but they overwhelmingly do --- presumably, not least because the primary input to the sentencing process is a PSR that is derived directly from the guidelines.

Note here that to reach 20 years, you have to do more than disagree with the guideline offense levels; you have to somehow disagree with the grouping rules. 20 years wasn't on the table to begin with (again: the prosecutors threatened a much lower sentence), but it couldn't seriously have been put on the table either.