A whole lot of depends. Time of year, time of day, number of people going to your resort, etc. In an ideal situation, figure 15-20 mins to deplane, take the monorail thingy and get to the DME loading area. Add on time for the further back in the plane you are, bathroom/food stops, etc. If you happen to time it perfectly, walk though the entire queue and get waved right onto the bus, another 30 mins from there. That is if you are the first stop on the bus. If you are the 4th stop on the bus, you may need to add another 20-30 minutes easy, never mind time if you have to wait for a bus to arrive, or wait for the bus to be filled up, those could both add another 10-15 mins. Then tack on another 30-60 minutes for the bus from your resort to the theme park.
As long as your flight is close to ontime, you aren't traveling during spring break type conditions, and you are cool with using the FP at the tail end of the hour window, then your 8 o'clock ones should work.