Strange, that was one of my biggest concerns about the car(being 6'2" 245lbs). I just drove back from Cali to Ohio and had no complaints, except a little stiff from lack of movement. I drove around 400 miles between gas stations at a stretch. Don't get me wrong, I wasn't in heavenly comfort like a nice Caddy, or Lincoln, but really for a sports car I was mildy impressed.
On the other hand I have a Mercury mountaineer, that gives me excruciating pain after only driving it an hour. I have no lumbar on my boxster or heated seats. I did have to move it up a little farther than I wanted to and incline the back as far as it could.
I guess you could always buy those beads the cabby's use.
Oh, i did have a sharp shooting pain at first but then I took my wallet out of my back pocket. Seriously!!