Do you mean where lumbar support would be? If so, what I did in my Seat Ibiza was to put some foam under the seat cover (but over the seat's foam base), it took a couple of iterations of cutting the foam to perfectly fit my back - but now all is well and you can't spot anything different.

 

I agree. I have an 04' e60 with sport seats. While they look awesome, I hate the lumbar. I would rather have less lumbar support, I hate having that bulge push on my lower back, but even when I "deflate" the lumbar all the way, I can still feel it pushing on my lower back. It's not really bad at all on normal short trips, but on a recent 5 hour trip, my lower back was killing me. I think next time I will put a cushion back there or something.

 

I find the seats to be very comfortable, but I've never had comfort seats in any previous BMW's either. I'm sure the comfort seats are much more comfortable (duh!) but the way I see it is - what I don't know doesn't hurt me.

I'm young (25 yrs old) so maybe that's why I don't mind the seats either. I was use to driving my Honda s2000 that had the most uncomfortable seats in the world. I since sold the Honda and picked up an '05 525i and an '05 Z4 3.0 and the seats make me feel like a king compared to what I was use to.

 

I have the comfort seats that came with my sport package. They are the best seats I have ever had period! I have a really bad back and one trip I could not stand up straight. I drove from North Carolina to New Jersey without stop and without pain!