We are close to being able to remap the standard ECU, which is great for mild power increases. Otherwise, an aftermarket ECU like the Apexi PowerFC is a great choice. Boost Controller and Exhaust/Filter upgrade would also be a good move.

You'll need a bigger turbo and injectors for 400bhp, but thats really pushing your luck on the standard engine, I've known a few to go pop at 400bhp.

Safer to aim for 360-370bhp IMO.