I'm just quoting from the Garrett website, but you're right, these are the power ratings the turbos are rated for, not what might be reliable for them or the engine.
That's good to hear that the GT-SS (-9) would be probably slightly more powerful than the R34 N1's (-7).
My impression was that around 300-330 awkw would be the sweet spot balanced between power and
- response
- driveability
- reliability
If you wanted to produce any more power the engine internals would have to be replaced/rebuilt at significant expense, and other mods like cams, fuel pumps/rails, intercooler, piping, radiator, plenums/manifolds, throttle bodies would need to be upgraded. The laws of diminishing returns kicks in as the cost rises exponentially for every kw you gain. And it would be a pig to drive on the street due to the turbo lag, unless I never leave 1st gear.