If it’s a VG then I don’t think much is different than the Catalytic Converters.
I’m not sure what the RB offers over the VG in the Z31.
I think Nissan used more as a stepping stone to offer the Z31 with different displacements.
There was the 2.0 Turbochaged. 3.0(VG) and 3.0 Turbocharged (VG30ET)
As for power I have no idea.
I suppose the difference in name makes some difference too.
200ZR, 300ZX…
The first three numbers represent the displacement. The Z represents the type of car or Chassis (Fairlady Z or Z31) and the last letter is the option level. X in Nissan nomenclature stood for Luxary at first. The R probably stands for RB motor which could mean that the car was stripped down and offered the RB with nothing else but performance.
As far as I can remember Nissan had a performance directive with all of the ZR cars. But then that blows appart my theory as a displacement stepping stone for Nissan.
Who really knows anyway??