I raised the back of the seat cushion to make it more level. It is still angled though because if I raise it to the point that the cushion is level, then that doesn't leave me enough leg room for my legs. With the seat all the way up, my legs are too long to be able to sit comfortably even with the seat all the way back.
However I haven't had any back pain so far so it seems your suggestion is working. Didn't drive more than 30 minutes today though.
2013 ///M5 Individual
Frozen Grey, Silverstone Full Merino, Piano Black, Exec Pkg, Drivers assistance pkg, LED Package, Bang and Olufsen, 20" Forged wheels