Sport seats differ in that there is an adjustable extension under your thighs and adjustable side bolsters (they move away or toward your torso) I could see if your big (read: wide) you might have a problem with the side bolsters not being wide enough. Better find some seats to try out. The sports seats are great to have, but ordering them and not being comfortable would be bad, bad, bad.

afraid this would be so. guess i'll have to wait till a dealer has one to try. z's are scarce in this area at dealers

I'm fairly wide and the sport seats in my 335is have been great, especially the thigh extender. The bolsters have been no problem, in fact I don't have them all the way out. My Z4 does not have sport seats.