I actually have experience with this on July 4th of this year- a busy night for firework viewing!
We had a 7:00 reservation, with Illuminations at 9:00. With appetizers, entrees, and dessert we did not finish our meal and get our check until 10:30. We typically eat here on our trips to WDW and get a patio seat, and even without an appetizer have no problem taking 1.5-2 hours for our meal. An 8:00 reservation is not unrealistic for a 10:30 fireworks show, especially since there are no reservations after yours. The staff will be there until Illuminations finishes so it is not prohibiting them from going home.
I will also add that on July 4th I was told (in spite of my having talked to a manager when making my reservation since I had a large group) that there had been groups seated on the patio since 4:00. For a 9:00 Illuminations.
I am telling you this for 2 reasons:
First, I would never advocate tying up a seat in a restaurant for a fireworks view for 5 hours. I just do not agree with that- it is inconsiderate to guests who have later reservations and should not be an accepted policy. However, if it is something they allowed on the 4th of July, you should not feel guilty making an 8:00 reservation for a dinner that will most certainly take 2 hours (at least based on personal experience). Staying an extra 30 minutes to drink an extra cup of coffee is not a outrageous, though I'm honestly not sure it will be necessary.
Second, be prepared for the fact that you may have to be seated inside and then walk out to the patio viewing area they reserve for guests. If there are a bunch of people with earlier reservations who stay on the patio from the earlier dinner seatings like they did on the 4th of July, you may not have another option.
Hope this helps a bit?