It can be done... all depends on how much time/money you devote.
Typical builds using the stock S4 or S5 block run a 60-1 turbo (BNR Stage 3), standalone EMS, streetport, supporting fuel upgrades, cooling and an upgraded intercooler yield about 315whp to 350whp at 12psi and 16psi respectively.
For 400 or more, that will come with a larger T4 turbo (or a highly modified, no holds bar twin setup... but only one person was crazy enough to try it
).
If you're willing to swap the engine, I'd run the 13B-RE or 13B-REW. Better ports and intake designs. Those motors with large single T4 turbos will make 400whp on stock intake ports and 16-18psi. Streetported and a good tune will get you over 400... the exact number depends on many variables.
Half bridge any of those motors to get another 10-20% power than a streetport, full bridge gets another 5-10% over the half bridge and thats as far as most people go.