400rwhp is about "the limit" for more reasons that just outright stregnth.
You have to understand the basic dynamics as I said. People with stock motors fit the following:
1. Generally drive the cars daily.
2. Do not want to rebuild @ a cost of 6-10k and indefinite time off road. Overall setup is budget limited.
3. They want good response and 400rwhp(280-300rwkw) is the "sweet" spot for power vs response and turbos like the 2835 or 3071/3076 fit in nicely
4. Don't want tyre frying monsters
You will not find "proof" of "many" people over 400rwhp/PULP/stock motor - because people don't want to push that fair as per above reasoning.
If you want more than 400rwhp generally people are racing and therefore go chasing 500rwhp or MORE. There is no middle ground there and it seems to jump markedly.
Read the dyno results thread you would see this for yourself.
FYI, I used to have a built 370rwkw RB25 back in '05/'06
As it stands, you have a cheap truck turbo that is worse than a old t04. If you put a decent turbo on there your lazy 400rwhp car would turn into one of the fastest 400rwhp cars around simply because the average power would be ridiculously improved.
If you want more than 400rwhp, then just get a GTX3076, E85, you should see 350rwkw easily enough.
Ensure you have adequate money set aside for the rebuild. As it will die, its just a question of when.