I'm inclined to believe this won't change the outcome but then...I don't know. But the fact the Comey came out at this late date perhaps suggests the evidence could not be suppressed. At least that's what Carl Bernstein believes:
We don't know what this means yet except that it's a real bombshell. And it is unthinkable that the Director of the FBI would take this action lightly, that he would put this letter forth to the Congress of the United States saying there is more information out there about classified e-mails and call it to the attention of congress unless it was something requiring serious investigation.And as Bookworm Room notes, maybe Hillary knows what's coming since she'd really like you to vote right away quick before the truth comes out.