It depends on how close to the front of the crowd you are, but assuming opening time doesn't get moved up to 7:00, I think 8:40 is probably cutting it a bit close regardless. On my first try I was nearish the front at 7:00 opening (not front of the pack but in eyeshot of Star Tours) and didn't stop walking all the way to the front of the extended MFSR queue, but my first photo after the ride is timestamped 7:54--I'm not sure exactly when I actually finished, but it seems like it'd be a bit close if I'd had anything scheduled for 40 minutes after opening.
Second time I ropedropped for a BG, I followed the exact same strategy and timeline as before and ended up way further back in the crowd, near the entrance for the Frozen singalong. The line was already doing stops and starts by the time we reached Droid Depot at 7:10-7:15, and I cut my losses and skipped Smuggler's that morning.
Unless you're willing to take a Lyft and get there very early, I think it'd be too tight. I arrived by 6:15 both times (having been at the AoA bus stop at 5:45) and it was really variable. Even on the morning it went well I think I would have been stressed about making it to the reservation. I'd suggest keeping an eye on park hours--if they bump opening back to 7:00 you can ride Smuggler's and make it to an 8:40 reservation no problem, but otherwise I'd suggest either hitting a different ride at rope drop or using that time to browse the shops in Galaxy's Edge. On the morning I abandoned the line for Smuggler's I actually had a really nice time poking around the market and so forth without having to fight the crowds. And if you're planning to build a droid, rope drop would be a good time to do that--even with a reservation it was really crowded at 10:00 AM when I built mine, but when I was browsing Droid Depot at 7:15 there was hardly anyone in there.
