We were able to reach 490 on overclocked GTX 1080. We also noticed that temperature has some effect on all pascal cards - the higher it is, the lower it would clock and thus slower it would mine. But the hashing rate was always consistent, never more or less, as long as all clocks were fine. Also, if there is a driver crash due to too intensive clock, it may put your card into locked low clock and reboot is needed. That is all I can help you with. You may observe GPU load, TDP, memory load and see how these numbers compare to your first run of 500+.