Yes, I'm pretty sure you've got the same seat on your SEUC as on my SERGU. I'm not sure about what you mean by "stiff"...if you mean firm, then yes, it is firmer than previous HD seats I've sat on, but for me, that is a good thing, as Corbin seats are VERY firm, but once broken in, there is nothing better for my butt. This new seat seems to be comfortable thus far for me...However, I will agree that the distance between the backrest and the TP backrest seems to be a bit smaller compared to my '06 SEUC. I've got my driver backrest in the second holes back from the front of the fender bracket, and the backrest in the top holes on the vertical part. I'm sure you know that there is an angle adjustment for the backrest. I'm not sure what makes there appear to be less room for the passenger...perhaps the lumbar adjustment in the passenger backrest? I also checked to be sure they had put the TP in the proper mounting holes, but it is. It's hard for me to judge, as the '06 SEUC had the smaller, leather TP, so this is my first with the King TP, and it is definitely about 1/3rd bigger overall.
I did have the driver's backrest as far forward as possible, but it seemed to push me a bit too far forward in the seat, and I'm short with short arms...moving it back a hole, and putting the backrest in the top holes, plus angling it forward just a bit, seems to give the passenger the same amount of room, but more room for her legs to be a bit closer together because the backrest is higher...hope that makes sense.
I'll soon know, as we are about to make a 300+ mile ride to Ribfest next week (her first real trip on the bike)