The BMW Z3 was a quick and nimble roadster. The 321hp I6 engine didn't leave its owners wanting either. However, BMW once built a V12 Z3. The V12s that BMW was using during the Z3's production run were a 5.4L unit that made 322hp or the later 439hp unit. We can't see M's engineers bothering with the earlier V12, so we're assuming that this car had the beefier powerplant.
It's a real shame this was never produced.
[via BMW M]