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I also have a lower back problem and the seats in my new 2012 are more comfy than in my old 2011 but... not enough in itself to justify a change just for the seats.
I am happy with the 2012 power seats partly because of the adjustible lumbar support, which I find effective for reducing my lower back aches. I usually readjust more than once if I'm in the car for any length of time. I also highly recommend the memory option to move the seats back for entry and exit.
Bottom Line: Definitely fix/upgrade your seats. It's inexpensive. And with the above parts, any decent auto upholstery shop should be able to do it in a few hours.