My Stock classic seat is PAINFUL!
I also have a Bonneville T100 with the factory King/Queen seat on it, and have done 1,000 mile days in the past; I can't imagine the same thing on this classic III.
It feels great in the driveway, and for the first 30-40 miles, but after that, I'm doing the sore-butt bugaloo. There may be one other factor, and that is the bonneville has highway pegs, which allows me to change back & forth between 2 riding positions. I've got Rivco highway boards on the way, so I will eliminate that as a factor soon, but any help would be appreciated before I go & spend big bucks on a Corbin or other aftermarket seat.