Graduate school applicant visiting a department he has applied to - advantageous?
It may not give you an advantage at being admitted, but it could help in other ways.  I visited my first-choice program (also in physics) around Thanksgiving of my senior year, and scheduled meetings with two PIs I wanted to work with.  The first stood me up, but the second took several hours to meet with me and show me the lab, and at the end of the visit he told me "If you get in and want to start early, let me know and I could probably give you an RA for the summer".  This ended up being a big help -- the visit made clear who would be better to work with, and also helped me get started with research shortly after graduating.