If the 350Z runs in the GT300 class (300HP) why do they turbocharge the engine if it already produces close to 300HP stock same with all the other cars. Like the ferries who run in GT300 but have more than 300HP stock what is up?

they have to run restrictor plates in there intake, also the weight to hp rate, also they dont use the VQ engine on the JGTC cars, its a modified VG30det i believ. there are alot of rules as to hp levels.