Fitted my second Corbin seat at new years(I has one previously on my America) and there are no performance issues whatsoever. All of the racks etc fit as normal although the stock pillion seat will not fit behind the Corbin seat so I will keep the stock riders seat for the rare times I ride two up with my daughter. The Corbin seat takes a little while to break in to your shape but even out of the box it is 100% more comfortable than the stock seat. I am 6'2" and I find no real difference in the riding position. I have the doughnut style backrest and it is a good size so that should not be an issue.
Hope this helps.
Thanks Blindaussie, I have a ride in appointment with Corbin in Hollister California this Thursday. It's a 15 hour return all Interstate journey with a 50 percent chance of rain on both days so I will sling my Bird along on the Kendon trailer I bought recently.
I am hoping to get a more custom fit with the ride in service. I will post my findings of the service and the saddle in due course.