The first thing I would say is...good luck with a GRR FP that early in the day! I've seen them giving out FPs twice...both in the absolute busiest times, and only when the wait times are already up at an hour. They seem to be *very* hesitant about opening up the FP machines for some reason.
But if you are among the first at the gates, among the first in the park, get on Soarin' (which would probably be a walkon), then get up the hill, you would probably hit it while they are still holding people back at the rope. So when the rope drops, if the FP machines for GRR are open, snag one and hoof it to TSMM, and I would possibly say a 20 minute wait, since you're slowing yourself down.
If for some reason you don't get up the hill before ropedrop, definitely 20, maybe longer.
That's in my experience at least!
The other wild card is when opening is! I've read that they are opening DCA earlier, or talking about it...and that's going to change up the early opening time, if they are going to have that anymore...