Wiesmanns were meant to be new healeys, then some of the people in bmw top boards, decided to have own factory and build them. That is why those cars get all the bmw technic they need, including smg2 transmissions, new 750/X5 4.8is engines and so on.
I guess their greed have denied us from having M edition of Z4 so far. AMG slk55 might help us and the Z4 M edition has been testing at the ring. Well there has been also other difficulties at bmw, too many top models coming out at the same time, all timelines are postponed, think M5, 130i, 4x4 5-series, 258hp engine in Z4 and so on. Only the M6 is within the waited timeline, but the price is imho too steep.