Blue Bayou is a table service restaurant, so no matter how quick you eat, your time there is dependant on the service. I have done the BB Fantasmic package in June, we had a 6.30pm reservation for the 9pm Fantasmic show. We were there just over the hour. Blue Bayou is an experience restaurant, meaning its not just about the food but the whole experience of being IN Pirates ride.
Also for Paint The Night people are generally in place at least 1 hour before.
Taking into consideration the walk from BB to Main Street, and the fact that you will be at the back of the crowd and wont have a good view, I would not attempt to leave New Orleans Square after your meal.
The tickets say go to the Fantasmic viewing area 15 minutes before the show, but if you do that you might not get a good spot. We went to the Fantasmic viewing area about 1 hour before the show. Don't forget people from River Belle Terrace are also in the viewing area, so the spaces can fill pretty quick.