All cards have the same limits for now. It may change in some time but without editing BIOS it won't make big difference which card version you have.
All GTX1070 and 1080 are overclocking at about 2100MHz +/-50MHz depends from luck. GTX1080 are overclocking a bit better than GTX1070.
Voltage adjustment is right now +/- 0.03V so it barely affects overclocking. Some manufacturers may set that higher voltage as stock.