Dining Package guests sit in the two center section of the theater. FP section is adjacent to Dining and FP guests fill in all sections to the left of Dining. If you are seated in the far left area, Beast section, it isn't ideal. It's a completely different visual experience than the central Dining area.
It is not that unusual for Fantastic to reach capacity and to have the gates close before the show starts; particularly if there is only one show that night. SRO is even more common.
Itinkso has described the different seating sections. With the package voucher, you don't have to be in your seats til 30 minutes before the show. I much prefer to use my 3 FPs for attractions during the day, and having a chance for a 4th. I really don't want to use my tier 1 FP on F. But everyone has their own priorities.