We've done this, and its doable, but the timing isn't leisurely. Once with Spoodles and once with Flying Fish.
With Spoodles we had 5:00 reservations and a 7:30 Fantasmic. We really wanted to be there 45 minutes early for good seats, which meant walking into the theatre at 6:45. That mean one hour and forty five minutes for dining and travel time. Remember the Fantasmic theatre is another five minutes into MGM.... The meal was a little too leisurely (appitizers and entree), and we ended up skipping dessert and grabbing a cab - but we made it in plenty of time - We were probably sitting by 6:45 - our goal time - or a little later (we were also traveling with two Seniors and two preschoolers, which made for more frequent potty breaks and slower travel).
Last trip we did the same with a 5:30 Flying Fish reservations. We told our waiter that we were trying to make Fantasmic - and he was wonderful about keeping the food moving through. We wanted dessert, so this time we skipped the appitizers. Two courses, served and eaten with speed (the meal really deserved better than we gave it) and another $5 cab ride over to MGM. However, this time we were walking into a nearly full theatre fifteen minutes before show time and our seats were not nearly as good. We didn't have the Seniors along, and the preschoolers are now in elementary school and can make good time if they need to.
My hints would be to plan on a cab (catch one at the front of BW) for $5-6 dollars. Tell your waiter up front what you are trying to do. Stick to two courses. Don't try this with people who are mobility impared (including strollers).