I haven't gone to Uni/IOA the week after Christmas normally, but I drive by it every day. Traffic that week is insane, get to the park 30 minutes (or more) before open. If you wait until 9:30-11:30 you'll be waiting just to get in the garage, through the line, and in the park.
Islands closed once or twice last year for capacity. (It just means they stop letting people in the park until enough have left). The Studios side can handle crowds a lot better, though. But with Diagon Alley they may even have to capacity close it there. Using the ticket system to just enter Diagon probably will be going on during that time as well. Diagon hasn't been using the return ticket system since about a week after the grand opening, but the Christmas crowds are beyond crazy.
Thankfuly Diagon can hold more, Gringott's will still have a huge line, even if it's working on all cylinders.
With the new potter area open and the Hogwart's Express, it's really hard to say how this Christmas season will be.
Christmas week is doable if you get there early, and don't try to do everything.