I have 2GB RX 560 and 4GB RX570 and neither mine worth a crap unless in Compute mode and with custom BIOS and/or clocks/voltages.

Also note that MSI AB stops working correctly if more than 3 AMD cards are present (no such restriction on Nvidia). Or, at least, that has been my experience. So, you have to use the much less friendly (but more capable, really) OverDrivenTool to tweak clocks/voltages. Also don't forget the infamous Pixel Clock Patcher to get Windows to recognize your cards if you do load a modded BIOS. AMD is a real PITA to use, but worth it for Ethash and Cryptonight (forks).