In the cookbook I just got for Christmas, there is a recipe for beef brisket that you cook sous vide (in a vacuum sealed bag) at 145° for 72 hours. I think the key to what you are trying to do is maintaining a safe temperature. You want to do the cooking in the bag, then sear the meat. For a rib roast, you would want to have the water at the desired internal temperature, and use a popup thermometer or something in the bag, so you know when it is done, then use a blowtorch, or hot oil in a pan to very quickly brown the outside. (It says a roast over 2 inches thick could take over 5 hours) That way the whole roast is a perfect medium rare, or whatever temperature you are going for, and you still have the sear flavor on the outside. It also looks nicer browned, of course. I would probably follow the advice of experimenting at some other time, but if you have any other questions I could answer from my modernist cooking, sous vide book, I would be happy to. I'm just trying sous vide for the first time today, so I'm not an expert, I am just relaying info from a book.