I think that in my experience the park also opens before 9 (8:40/8:45 ish). This means that there will be hoards of people waiting in a massive sea of confusion and frustration until the rope drops and more people fit thru the turnstiles and fit into the bag check area.
If you REALLY want to take advantage of being on Winnie the Pooh before the lines pick up, you need to be in the middle of that sea of confusion, not the back of it. If it's not a super crowded day for MK, I have still done very successful rope drop starts arriving at 8:20/8:25. If you can pull it off to arrive by then at MK (not TTC), I think it would be worth it!
To help understand time - I would figure 5 minutes to park, 8 minutes to walk from car to the bus, 10 minutes for the bus, and 5-10 minutes to navigate security if you are all folks that can navigate 'with purpose'. In all likelihood - it could take longer.
The advice to take the less interesting bus is a good one - the lines coming off the monorail are the worst and God forbid it get stuck!