I'm not saying the Z4MC is the optimum track weapon in standard form, but I was really impressed with mine. I'm the first to admit though I am a relative novice and the DSC was on, but the car behaved very well, the brakes didn't fade on a circuit that has some heavy braking (although they were starting to groan a bit) and the grip and overall handling was really good. It also hit 120mph on Park Straight before braking and sounded superb too.
If I do a few more track days I might do the brake upgrade, engine mounts and suspension, but other than that I'm not sure I'd do a lot else.