I have driven both of these cars, I work at a toyota dealer and my best friend drives a z32 w/ JDM vg30dett swap. The engines have about the same power potential it's just more common to see a 2jz built to the higher end of the spectrum but it's fact that the fastest Z will destroy the fastest supras on the drag strip(escort engine service z32/Titan motorsports supra drag car) and road coarses, nissans most succesfull race cars were powered by vg30's such as the three time IMSA gt Champ NPT-90 and the GTP-ZXT. The VG30 is also a more advanced powerplant implimenting vvt long before it was available on a JZ motor and due to its over square design as apposed to the 2jz's square design it is much happier at high RPM's. The biggest player in terms of performance difference though is the Chassis/Suspension design. The Z is superior to Supra in Terms of Handling(agility) and ability to put the power these vehicles are capable of when modified to the ground, check your local drag strip to witness 600hp Z's smoking 800hp Supras. There is truth to all the jokes you hear about 1200hp supras not running even elevens, this and more comming from a supra fan who is willing to be honest rather than indulge in automotive elitism.