Mine topped out at 330hp with the factory airbox and paper filter.
With an aftermarket filter, 360-370 is achievable. I'm basing that on a dyno tuner that's probably tuned close to 100 of them, so the data is reliable. That is from a stock K04 engine running 1.5 - 1.6 bar boost with 3"+ downpipe and uprated HPFP. The factory cat back doesn't really restrict flow that much, so a full system is down to personal noise preferences.
400 isn't doable with a K04. Well, you might be able to see that once or twice on a cold dyno pull pushing it to 2 bar, but after the 5th or 6th run it will drop right down again, and 2 bar isn't sustainable.