Yeah the wheels are pretty small, there are no signature M bits, and it could very well be a Z4 with a bodykit. But it's also possible that the guy driving the thing is like, a known BMW test-driver or something.

As for weights, the '02 S54 M Roadster weighs 3131 lbs and the '02 Z3 Roadster 3.0i weighs 2910 lbs; so that perhaps gives an idea of the weight increase that would go along with an M version of the Z4.