I think it depends a lot on how many people are in your party, what the EMH park is that night (if any), how the weather is, and how many shows of Fantasmic there are that night (1 or 2).
We have found that showing up about 30 minutes before the show, we have just 3 people in our party, has been sufficient the last couple of times. However, we would not have been disappointed if we couldn't have made it.
If you have your heart set on Fantasmic, I agree that you should either go with a dinner plan or show up about 45 - 60 minutes early.