Try a chip. Bavarian Autosport, Dinan, and Autothority (in addition to many others) all ought to have chips for your car. At
www.bavarianautosport.com, you can input your year and model, and all upgrades and accessories for it will be listed by category (performance, interior, exterior, etc.). Depending on how much you want to spend, you could easily get 220+ hp from your car. A chip alone (good for 200+ hp) would probably be a relatively inexpensive $300 or so.
www.dinanbmw.com
www.autothority.com