Strong Strut - strut braces
Before doing this I was very skeptical as to whether or not you can feel a difference with the strut brace on the Z4 M. I would have to say that you can most definitely feel the difference.
This item cost me $395. It was easy to install, although it will require a wheel alignment because you are unscrewing the strut bolts. i recommend you get the textured finish, as this couldn't match better if they tried with the plenum, airbox and valve cover.
I suggest that if you do this that you also knock out the strut pins and get some free negative camber prior to getting the alignment. Trust me, its cheaper that deciding to do it later and having to get a second alignment......at $85.....