I had the XT-780 but now I have the XT-785 trail. The trail is better than the 780 (the bigger platform helps), but neither come close to SPD-SL in stability and long distance comfort (more than a hour of use).
My wife had XT-780's on her bike for a while because they are easier to get in and out of. About 6 months back she bought a pair of Look Keo 2 Max pedals and she too said they feel a lot more stable and comfortable over longer rides.
Interesting, I just did a tour with Speedplays and I had to unclip a lot because the group was so slow. Plus walking around in Speedplay cleats even with Keepon Kovers is not the greatest feeling in the world especially at the end of the day when you need to pee and the restroom is in the middle of an ancient Buddhist temple (don't ask) and you can't take your bike in!
For my next trip I was thinking of switching to XT-785 pedals as they have a wide platform, and I can get the same shoes in SPD cleats (Giro Empire / Giro VR90) plus walkability of MTB shoe. You're saying that SPDs aren't comfortable for more than an hour? Or is that just your personal take? I've done touring on 520 pedals before and hot spots did come up after awhile, was kind of hoping the the 785s with a wider platform would solve that plus a more rigid sole.