In our testing we've found the N55 is an awesome engine that is somewhat hindered by a tiny turbo. FBO + E85 cars are good for ~380whp and TONs of torque(460wtq+.) On pump gas 350whp is doable.
Keep in mind that dyno numbers are subjective and can vary wildly. Find a nice, optimistic dyno and 400whp may be possible but that doesn't mean it's any faster than a dyno reading 380whp.