Hard to imagine we're at a point where we're asking if 400 HP is enough. My WRX is only rated for 268 HP and that's at the crank I believe. Wheels are more like 220 HP. Without launching it (getting into first gear lightly and then hammering it after) I can get it to 60 MPH in 6 seconds. For a daily driver I think it's pretty good. Can't imagine nearly double the power. It's already quick enough to send me to prison.
I think after a certain point HP doesn't matter, but how the power is delivered and put down is far more important. It's why a GT86/BRZ is able to keep up with a Cayman in the corners.
I'd rather take a BRZ with my WRX's engine than a 500 HP boat.