These folks, in Germany, have been able to get 281HP out of the 30i. I have only seen one installation, with 25,000km on it and so far no issues, but they don't have a bad rep over here. Not sure if they offer reprogramming or an external "tune box".
http://www.chip4power.de/de/fahrzeugauswahl/PKW/BMW/Z4-Serie-E89/Z4-sDrive-30i-258PS/
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