Before dinner.
In fact, assuming the coaster does remain operational for RIP guests (good luck to them if it doesn't), I think that's the only path that makes any real sense. The dinner is generally the first or second stop for most (if not all?) RIP tours, regardless of starting time, which are staggered to of course avoid overwhelming any individual attraction (including the buffet).
Fast & Furious is located directly next to French Street, which is where the buffet dinner occurs. I assume it'll become the first stop, with dinner immediately following, and then the normal flow.