I struggle too on the long trips. I've always had back issues so it is expected in most cars. I'm 5'7'' 165lb and the previous owner was much bigger and the bottom cushion is a little softer than it should be. There is a metal or plastic bar very low down in the back of the seat and it can cause a sore spot low on a back. I put a folded up towel on the bottom cushion to raise me up and it helps a lot.
I've driven in a friends Cayman R for hours and been pain free, very unusual for me. The seats are the sport ones and I just love them. I found a pair to put in my Boxster but they want $3200
. He now has a 991 with the fancy seats and I find them too big and not as good as the Cayman seats.
I'll probably bite the bullet and get the sport seat at some point because I would love to drive to the mountains once a month but just find it too hard. I'm wondering if the standard 987 seat would due but I'm not sure...they certainly are less expensive.