In my opinions Q60 RS400 with 245 rear tires is a joke!!!(very dangerous) The rear OEM tires in my 2007 G35S are also 245. Even though I put very sticky Bridgestone summer tires on it, I think their traction are still not good enough in high G or speed cornering especially in rainy days...
In USA every Q60 RS400(RWD) comes with 265 or 275 rear tires.
My wife's 2010 M35X has 4 very good Continental performance tires (all season) which are also 245. In M35X I still need to take corner much slower and more carefully than my G35S because M35X is heavier and all season tires are much less sticky than summer tires but all season tires can last much longer than summer tires.