There is almost no difference between 4GB and 8GB performance. My 4gb RX580 and RX570 cards with Elpida memory are doing 31-32.5Mh/s with good bios mods. It comes down to the quality on that specific card, some i can run at 2170MHz while others i can only run at 2080Mhz. At the same moment, my 8GB RX580 cards with Samsung memory only does about 30.5 to 31Mhs.