i work with seats all day putting leather on them (xterra's too). and there is a couple thing i can think of that might help using your existing seat. if you can find a car interior shop near you, call them and see if they can add some foam in the lumbar area under the seat covers to make it curve to your back a little more. you can also do that to the front of your seats to hold your legs up a little for you taller people. they should know how much they can put on without changing anything with your cover. the second thing i can think of, which i dont know off hand where to get it, you can get a lumbar kit for your seat. it goes under the cover and you can adjust it different ways. i believe you can get in electric or with a hand pump to add air.

hope that helps.