Rx 570/580 bios question

[venomcry]

Hello, i'am going to buy significant amount of RX 570/580 8GB Sapphire nitro+. I want to get as much performance from them as i can and surely i will bios mod them. I heard that there is some memory types that cant be bios modded - is it true ? Is there any difference in memory manufacturers regarding to performance?
What i should be warn about before buying these cards?
Thanks.