Stock they have 112kw at the fly, so about 80-90kw at the wheels.
That was brand new out of the factory, so I doubt you'd be making more than 80wkw 20yrs later...
RB30s are no different to any other NA motor, so anything that increases the basic needs of an engine is good (more air in = more fuel in = more power)
As for what you can and can't do on your Ps, I havn't a clue, I live in WA...
If I was going down an NA route, I'd be looking at a good free flowing exhaust (2.5" would be ok) including extractors.
Then it can get VERY expensive quickly, you COULD go for an RB25 head, higher compression, fat cams, higher redline, and a gosh darn-off aggressive tune etc but it depends how much money you've got to throw around.
But it would never be worth it, cos all that would result in eff all gains.
Save your money and buy something with a better platform for power production when you're off your Ps. i.e. something big (>5L) or turbo...
[edit] look at post no.7 (from gts-143) on that first link that TwoCents posted... That's a perfect example of a well done NA RB30 engine...