Aerodynamic drag on a vehicle increases as a cube of the % speed increase, so the extra power required to reach a certain speed increases in proportion to that.
32 GT-Rs do about 265km/h stock, so 310 would be near as a 17% increase, so 1.17^3= 1.602. Which means you would need around 60% more power than factory to do 310km/h- Assuming that 32's have 310hp standard (common realistic figure), that would mean you need at least 495hp.
Baseline rwkW figure for standard BNR32 is 170rwkW im told, so 300 would be a 76.5% increase, yeilding a 20.85% increase in top speed, or (with appropriate gearing/rev limit increase), around 320km\h.
Hope this helps