For an ADR at 8am, at Kona, I would be out at the bus stop at SSR by 6:45ish. These early buses tend to run a bit less frequently than normal resort to park buses do. They start running every 15-20 mins about 60 mins prior to park opening.
So, be out by 6:45. When you get on the bus, ask the driver if it's faster to take the monorail or the boat over to the Polynesian from MK. The driver may very well offer to drop you off at the Polynesian on his/her way out. NOt always, but sometimes. If there is no offer, walk right to the monorail platform, just past MK bag check area. The monorail starts running by 7am. This should get you to Kona on the early side.
The latest I would get out to the bus stop would be 7:15...and that's taking a chance. It could be a 20 min wait for the bus, then a 15-20 min ride to MK, then wait for the monorail, and then 10-15 mins over to the Polynesian.
If you really don't want to leave SSR that early? Get a cab no later than 7:30.