The Z4M hasn't proven to be much faster than a properly setup E46 M3. It should be but since no one is really racing the z4 it's is way behind in development.
Due to the rarity of the Z4M and the lack of aftermarket race parts support I actually believe the M3 is a better track car. Some of the race prepped M3 are down to 2600lbs.
The Z4 does need a rollbar. If you are serious about a track car then a rollbar, seat, and harnesses are fundamental.
This is what a Z4M looks when you roll it.
