A big high flow turbo + E85 + Blitz return flow + factory airbox would keep you under the radar.
In the end police will always be able to find an ECU or injectors or boost controller if they want to look for them, which means anything over 170rwkw has the potential to get you found out. It's all about hiding it as well as you can. If you can keep the engine bay looking stock they won't have reason to look much further. I'd argue that any forward facing plenum is grounds to scrutinise the engine bay, versus a cooler that uses the factory piping.