As others have said there is not a huge difference between the 4Gb models and 8Gb. The Sapphire brands tend to reach 30-31 much easier than the other brands regardless of memory type in my experience.
There are huge differences in memory types... Some of the Sapphire with 24MJ can do more than 32MH/s in dual, while 24AJ has problem reach 31MH/s or at least with my timings... On XMR I was able to tweak both of them more and the difference is lower.
BTW few days ago I've got bios with H5GQ4H24AJ, I've never seen this type before, but it seems that it likes same timing as H5GQ8H24MJ and it does 31+MH/s.
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.
It pretty much depends on exact Samsung type, classical FB is great if you don't have some bad one on the other hand FC and FE are terrible, I need to play with them a lot.