400hp = 300kw = 250rwkw (approx).
On 15psi something is very wrong.
Now if you actually mean 400RWHP...
400rwhp = 300rwkw
15psi, that result is probably a little bit low, but put 20psi in and it should wake right up.
I have 365rwkw @ 19psi with some timing pulled out, stock cams/manifolds/cooler/airbox.
Also have tomei dumps, which made no difference when compared to other cars with stock dumps (other than the exhaust note change).