So I have the 18x7.5 sedan sports suspension wheels all the way around with the OE 235/45/18 tires. If I switch to a staggered like 18x8 front and 18x9.5 rear what are the benefits of this for performance. Obviously I can run widder tires, more than likley 245/45-18s front and 275/40-18s rear. Is the extra 30mm in the rear that big of a deal? For launching im sure it is a bit of help to have the 275's on the back. Obviously 245s will out perform 235's a bit in the front for turning. On a sedan having to run a really tucked in set of rims to get make the wider rims and tires fits seems like it might make turn in feel soft because of the extra tire in the back is more inboard? I dont want to deal with getting adjustable suspension arms just yet and If I lower it will be nothing more crazy than 350z springs. So is the staggered set up on a sedan a HUGE performance enhancement of does it just look cool?

 

I doubt I can give you straight answer-may be somone else will, but I will try based on my experience.
With wider rear tires you will get less wheelspin under throttle at the corner exit. However my impression of the G is that wheelspin at corner exit is not such a big issue as understeer(front tire slip) in beginning to mid corner. My unfinished tuning can be factor here too.
From my POV, wider rear tires are not HUGE enhancement because there's no HUGE wheelspin at rear tires. With very wide tires you start running into increased rolling resitance, more drag and they are more prone to aquaplanning.