That's what we did the last few trips! The fantasmic package restaurants seem to be VERY busy during the fantasmic dinner times. We did it once at Mama Melrose and twice at Brown Derby. Our lunch at Brown Derby (non-fantasmic package) was much more enjoyable another day.
We absolutely prefer SciFi Diner and 50s Prime Time for lunch or an early dinner and we just don't worry about getting a seat for Fantasmic. We always got a seat... that being said, we travel during less busy times of year and will choose days for Hollywood Studios that are expected to be less crowded during our vacation.
I actually like sitting all the way in the back (top) in the fantasmic theater -- it's faster to exit (or go get drinks, popcorn, etc.) and you don't get the water spray or feel the heat from the fire... On a cold night though, we were more than happy to sit closer! The fire on the water warmed us up!
If I were traveling during a busy time (Thanksgiving, Christmas, Summer, etc.), I might consider the fantasmic package again.