i bought the first rx6 kits in back in 2000, the original kit ended up being sold to JMS and went on danny's r33, which then got sold to mark phillips (legend too btw). Some of the best turbo kits around. But twins on the gtr (named the V-max SPL kit) is realistically 1000-1200odd horsepower, a single rx6 rb26 kit is good for 500-600hp
i remember sourcing the smaller rear to make it more responsive on mark car, i got to drive this setup at mallala one drift event (i was shit scared of damaging it so i sucked bigtime) and the car was a real weapon made killer power yet responsive to boot.
Have a look at the turbos as the rear housings sizes are p13 being the smallest to p27 being the largest.
The frame will be a TCW number and i do have some compressor maps for the TCW77, TCW76, TCW15 and the baby TCW10