With 225/45/18 and your gearing 180mph will come up at 7500rpm. Assuming your engine has a decent torque spread and is making 600bhp at the 7500 - 8000rpm then you should manage around 185mph.

As always though if you have'nt got the power you think you have, you won't make it! Many think they have, but in reality so few actually do! Top speed is more than just peak numbers on a dyno, it is about understanding what it takes to make a car hold full throttle reliably for 30+ seconds! Are you confident the cooling and most of all the mapping is safe for 30 seconds at full throttle!!