Should be... saw video on youtube where some guy upgraded his z4 to m bumper...
I even asked in comments if anything else had to be replaced (ducting, undertray etc). He replied that he did not replace any of it... and it did fit.
this is the video...
https://www.youtube.com/watch?v=_MXwh3CxddA
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2009 35i DCT Havana brown with ivory white interior. Mods: Cobb stage 1+FMIC, Wagner EVO1 FMIC, BMS DCI, KW V3 suspension, M3 front control arms, Hardrace RTAB, Michelin Pilot Sport 4S tires.