Well, I took some time yesterday to look at those seat backs, and it's like COCYclist described. The 1/3 part is quite easy (if you know how to do it, that clamp is well hidden), the larger 2/3 part needs more attention, because of the seat belt. Therefore the belt holder must also be removed, with a multi torx. With those backrests out, the entire trunk lining could also be removed. Now it has really been reduced to bare metal.
This isn't something I will do every time, but good to know that it's possible.
A quick weighing of those parts resulted in 24 kg or 53 lbs. That now makes a total of 75 kg or 165 lbs weight reduction.
My car now drives like a van. Thanks to the extra driving noises and the rear suspension that reaches for the sky and gets bouncy!
