The original question was about an L20et Vs an RB conversion?
Your right, any replacement engine bigger than 15%, although NSW RTA will allow an L24 replacement as the car came out with it in Aust, will need an engineers report if it is going to be driven on our roads.
MAGs comes out with "go the RB" because of costs, and most of us are suggesting staying with the L20et and making it work.
I would think, but not having done it, by letting it breath a little, the sky would be the limit with the only restriction being the 2.0L capacity. Maybe an N42 head would help (but need to check valve clearance to deck due to the 79mm bore) maybe change the AFM or do away with it completely and use an a/mkt ECU, or a piggy back to control fuel & ignition, change the turbo to a bigger T03, something a little more modern off the RB20 or 25 maybe, or the non intercooled FJ turbo and all this could be done progressively and not in one big hit.
As Stagefummer says 180rwkw is not impossible, as that's 240HP, a big increase on the stock 104Kw at the flywheel.
Cheers, D