I agree. I'm not sure that a voluntary research year would benefit your resume. It could be useful if you don't match (which isn't particularly likely given your stats), but dragging a year to to research isn't really going to give you a whole lot, especially if you haven't already begun a project, since you won't have any new pubs to show when you're interviewing in the fall. It's also a bit of a risk, because there's always the chance you won't produce much in that year, and because some folks might red-flag you wondering "so WHY did this guy pass and hit the lab for a year... should we be worried?"
I suppose a good indication would be if the PI was some high-powered guy/gal at a program to which you'd really like to match and you want the direct face time and haven't had it already.