You won't gain much. Figure on 0-60 in the high 5's stock, and you won't really gain much of anything by modding (except looks, sound). IMO, the 330i is powerful enough to be fun, but it's not a sprint car. That said, the NA engine in the 330i is a blast, sounds great, the chassis is absolutely superb, and the car couldn't be a better daily driver.
Here's a good long-term (40K mile) test summary from C&D: http://www.caranddriver.com/reviews/...term_road_test
(VanderWerp summed it up for all of us: "This is simply the best daily driver on the market—the ideal blend of sport, luxury, practicality, and excitement.")