900..sounds like BS...lets see, a G35 has a 3.5 liter V6...for it to make 900 horsepower, even with twin turbos, it has to be running well over 30psi of boost...there is no way you can run that much boost on PUMP gas, you need race fuel. There is too much detonation on 93 or 94 octane gas to support that kind of boost. So, if the G35 owner was filling in anything less than 100 octane, he is not making 900 horsepower in his current setup.... Now, it is possible that he has some race tune that pulls 900 bhp at 30 psi of boost at 116 octane and 500 bhp on regular pump gas and low boost......in which case he needs to specify that!
You can't determine the amount of boost a specific car is running by displacement, to achieve a certain amount of hp. Head design plays an import roll......
In my personal experience, I made over 600rwhp and 650lbft on pump gas with 11:5:1 compression.