I work for a large Jeep dealer and have had lots of experiance with lots of different kits some good some bad, However the only system that would be any use on a 300ZX should you wish to do it? Would be the seqencial systems as they use the vehicles original ECU to control the gas and fuel flow switching between the 2 as required for more power etc. Gas will reduce the power by upto 25%. System will cost £1600 and i think the Goverment don't support this car (not 100% on this so check first).
Personly the experiance ive had has been on avearge poor with lot of Engine managment lamps consistently on and needing reset etc. Lack of boot space after they install your tank and lower performance. Also there are a lot of companys popping up and fitting systems and then going pop just as quick !! All in all i would'nt bother You got a "Super Car" keep it that way !!