If you are going to install a more aggressive Front sway bar for Track use ( as many do) read
THIS thread and
THIS thread
If your car will be mostly Track, the Clubsport coilovers, See
THIS Thread and
THIS thread
If you have searched, you've probably seen all of these.
The HACK says
THIS Code change is the best Track Mod you can do to your car.
Not many Z4MC's out there, fewer yet being tracked.
Good Luck! Have Fun!