Seriously they sound like dog shit. Don't put 3L in it if you only want 300-400kw. The cost to do it for what you want is nowhere near worth it.
And whatever you do don't stroke the original motor in a vspec. Keep the motor standard specs and go from there.
-9's, -5's, -7's will all make it in the power range you want. -5's will be the higher of the range.
Have you even been in a properly setup GTR?