[Pierre]

I layed both subframes next to each other. Boy, what a difference. The M roadster subframe is more substantial. It's beefier, the mounts to the body are stronger, the control arms are significantly stronger, the rear hubs are larger, the half-shafts are bigger and the M brakes are huge.
Ideally, you should use an M roadster diff too. I was not so lucky to find one, at least not at a reasonable price. I am using an E28 LSD from a 535is with 3.25 ratio. It bolted right in and the driveshaft hooked up to it with no problem. I did have to use output flanges from an E30. They sanpped right in and bolted right up to the M roadster half-shafts.
I do not know if that holds true with the regular Z3 with regards to size differences with the ti subframe.