seats are almost as individual as the choice of tires or oil.
that said, the folks that do lots of miles on their bikes typically use custom seats. I've had a custom seat made for my last bike, Rick Mayer in California made it and it was indeed comfortable. I was able to do over 1,000 miles in a day, and many days over 600 miles, consecutive and otherwise. That seat was pretty hard. The seat on my 919 is stock, it is pretty firm as well. I'm not probably going to jump at a new seat just yet as I'm using the bike for commuting. I will likely test it on some longer rides and may change my mind then.
Sargent seats make a pretty good seat as well, I had one and it was pretty comfortable. I've never tried nor had a corbin and can't comment on them. Believe it or not, some long distance riders swear by the wooden bead seat covers, like the cab drivers use alot of times. I've never tried one of them either but would consider trying it on the stock 919 if I was going to ride it far.