Thats right as far as i am aware.
The Z3 uses a nearly identical rear end to the E30. Except with 5 stud hubs, bigger brakes, some new diff ratios and the option of a very tasty 'torsen' diff.
The diff is indeed the same except with that nice finned rear cover. This cover should fit any other similar aged medium cased diff. E30 E36 E34 (small engine 5 series) etc.
Any more questions??
Chris
Mind you, i am just about to go and double check about all this.