I had the Dinan flash in my 335 for 40,000 trouble free miles. The N54 motor has great tuning potential. It is easy to get an additional 80hp and 100lb/ft of torque with a flash or a piggy back computer. It totally tranforms the car. I am waiting for Dinan or another tuner to design a flash specifically for the Z4. I have a JB+ installed in the mean time.
http://www.burgertuning.com/products.html