I don't think it's as much seat design issue as a seat <-> person fit issue. Different torso and leg heights, different physiques, different weight--it all makes the fit different. Think about it, we all have different shoe sizes and widths, but stock seats come in one size only. And the Z4 doesn't have adjustible lumbar (or adjustible lumbar positioning up/down). A good professional racing seat is more or less custom designed for the intended user. So that makes the suggestions for lumbar support, and other changes to the seat are good ones--it's about getting a good fit.
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