From what I have heard, BMW isnt releasing an M version of the Z in any near future. They really dont have a motor to put into it right now, the e46 m3 motor is stopping production within a year or so, the e39 m5 motor is no longer being produced, and I would think they wouldnt put the e60 m5 motor in the z4 because of cost and weight
You can get a hardtop for the z4, which makes it look like a coupe. Its a sculpted roof that actually looks pretty nice.