depends, what kinda of 400hp.
400hp with stock internals or reliable 400hp with forged internals.
Obviously an opened motor would be less reliable and the lifespan of the engine
would drop drastically if pushing 400hp.
400hp, you'd need.
HKS GTRS or bigger.
aftermarket exhaust manifold
Tomei poncams atleast (256)
rocker arm stopper
1.2 mm metal gasket
ARP headstuds
electronic boost controller
injectors : 740cc or bigger
255lp/h fuel pump, Bosch 040/044 or Walbro
Standalone ecu : Power FC, Haltech, MoTeC, AEM, etc
and a few other things I can't think of at the moment.
aftermarket valve springs and rockers not necessary needed unless you'd plan to run 256 and above cams.
How much boost you can run on stock internals varies on a lot of things.
There's so many factors involved, such as the tuning (ignition, fueling, timing, etc)
Boost itself is another factor, a 1.5bar from a GTRS and a 1.5bar from a GT3037 Pro S is very different.
Weather temperature = climate, would make a difference.
How long would you run that 1.5bar ? 1 minute ? 10 minutes continuous track/drift work ?
Heat can really kill an SR, good thing you already have an aftermarket radiator.
You should also think of a oversize sump, oil cooler, thermostat and a breather tank as well.
cheers.