I would strongly doubt that any criminal conviction would come of this, honestly. The bar for criminality in this would have to be pretty high, and if the review is based off of leaked Podesta emails then it's pretty damned benign.
The bigger story here is the public perception angle, of course, and while this could lead to tightening in the poll numbers, I don't think it'll do much to swing voters one way or another. This is a super polarized election cycle already, and anyone who vehemently hates Trump or Clinton isn't going to have their mind changed by this.
I'm calling a 3-5 point Clinton win, even after this story has time to sit in the public consciousness for a while.