If your only complaint is the stock pads and the stock seat belt, I'd say the car did pretty well.
I had my first track day with the Z4MC last week at Watkins Glen. I'd been tracking an '05 Cooper S, so there was a bit of a learning curve involved (lesson one: don't trail brake at Turn One until you're completely intimate with the chassis dynamics). I had a bit of a problem with the brakes - new Cool Carbon pads weren't agressive enough to scrub the rotor clean of the OEM dust, so the right front would squeal and shudder a bit after a few laps. Of course, I left my new, in-the-box Carbotechs at home. Once I get the brakes dialed in, this thing is going to be a monster.