OP says 500hp then a 500hp turbo will get him there. Do you agree with that?? (ignore the slippery slope that is drivetrain losses and has been argued to death in forced induction since 2003)
GT3076 with bigger fuel pump, bigger injectors FMIC, 3" exhaust, FMIC, engine management + tuning is limited? In what way? What else is needed?
Going over 500hp with a stock engine generally becomes far less reliable so the costs start stacking up over this level and you won't get an argument from me on that. Tuning needs to be fatter and less aggressive to keep well away from det.
I have owned an R33 since 2001 and the Stagea for quite a while too. I strongly doubt you have spent more on them than I have over that time.
To get your 700hp is an expensive road to go down there is no question that engine internals, balancing and presumably a good look at the head is needed to get a reliable result. Then you open the door to drivetrain issues and the spending mounts up quickly.
On drivetrain losses, I believe you are making a mistake if you trying to use % loss. This has been argued back and forth in forced induction. Consistently the S1 GTS-t's make around 125-130rwkw in stock form (ignoring some outlying dyno variation). It shows around 60kw loss. AWD Nissans generally show around 10kw additional loss in AWD vs 2WD but there isn't as much in it as people think. Auto losses are definitely higher. Most cars show largely fixed loss (and if you don't believe me dyno a car before and after NOS and see what happens) but it makes sense that the harder you push the car the more will be lost to friction etc. IMO there is a fixed loss with a slight increasing % change at higher power levels.
Getting back to the OP's goal 500hp fly is easily achievable. If using a slushbox that needs to be looked at.