There are saddle pressure sensors and people have done tests with rocker plates. It reduces pressure hot spots. I noticed with the rocker plate that if I'm seated with relatively low cadence it rocks more than I would expect on the road. It's just physics, the forces are different.
I think most people have something like 5% higher ftp while standing. I imagine it's because you can engage your upper body more. Of course, that has to be trained. One year I emphasized standing a little too much on the trainer and couldn't climb in the saddle nearly as well.