Whether the 15 minutes is worth it to you will depend but you can decide based on my experience below:
On my trip to TDR in February, I arrived at the gates before 9AM for 10AM park opening on both days. It was sprinkling in DisneySea and I was well behind bag check. Once I finally got into the park, my first FastPass for Tower of Terror was 11:25 and all major attractions were already over 30 minutes of wait. The next day, it was snowing at Disneyland so with fewer people, I was close enough to the gate to kind of see the characters come out for gate opening. My first Fastpass for Space Mountain was 10:50 and I managed to hit Winnie the Pooh at 5 minutes of wait time. When I exited the ride, it was at 50 minutes and in the afternoon the ride reached 150 minutes.
People with the 15 minutes privilege who arrived 30 minutes later than me could have:
1. gotten 2 more rides than me at DisneySea due to the first walk on and another Fastpass due to the earlier next Fastpass time
2. gotten a better spot than me for the gate opening show at Disneyland (and stayed out of the snow for longer)
I'll disagree with some of the people here for Fastpass strategy. Nemo SeaRider was the second to last FP to disappear when I went to DisneySea and became a walk on at the end of the day. However, you may want to ride it multiple times though because like Star Tours, there are multiple sequences but I still don't think it's a good first FP. If you want to do Midway Mania that should be your first FP. Otherwise, get Tower of Terror (you don't want to miss that). Your Disneyland strategy can vary but if you can get in fast enough, I think Monsters Inc FP then riding Winnie the Pooh is the way to go. Alternatively, get Winnie the Pooh FP and then ride Peter Pan (which isn't too different from other parks though).
Single rider is more powerful in Japan than in the US so you may want to use this to cut out some FP. It's hard to recommend rides because people have different preferences but I think someone with access to other Disney parks would find the following unique enough:
1. Winnie the Pooh
2. Nemo SeaRider
3. Aquatopia
4. Space Mountain
5. Sinbad
6. Monster's Inc
7. Jungle Cruise
8. Tower of Terror
The Electrical parade, Fantasmic and One Man's Dream are excellent shows. I also highly recommend eating lunch at Cape Cod Cookout which which has two short shows: one about ShellieMay and one about Gelatoni. But I'm a bit biased given that Gelatoni is one of my favorite Disney characters.