Just to confirm what others have said, just last week someone rode the first sub, and they said that they were finished with Nemo at 7:30.
Here's my 2 cents.

It's really hard to say what the lines will be like at 7:45 or so, but it might be a better use of your time to get ready to get a FP for Space Mtn or to line up for the "rope drop" to get a FP for Splash. This is just my guess, but if there are a few minutes left before 8:00 the lines might be short for the King Arthur Carrousel, Pinocchio, or Snow White.
Whatever you decide, have fun!