For an 8am ADR, at GG, I would plan on leaving the Polynesian by 7, not 7:30. Early morning buses tend to run less frequently than the later buses. So, if you arrive at 7:25 and the bus to Epcot is just leaving, you may not see another one until 7:45-7:50. Then, it's going to take you about 10 mins to walk from the bus stops to security and get through. Then, up to another 10 mins to walk to GG! You need to leave an hour....to be safe and unrushed!!!
I would actually ask a bus driver the day before you want to go. They're the ones who have the best info. If there is any doubt a bus might not be there at 7? I would take a cab to Epcot...either the front gate or the Beach Club and use the Intl Gateway to enter.