If you have the same card, you actually have to take into consideration 2 aspects
1º the asic quality between the cards (silicon lottery)
2º quality of ram modules.
That said, 100mhz on the memory will not give you 2 mh/s more, but good memory timings yes. also gpu core speed is very important. Just one example: if i reduce the core of one of my RX 580 sapphire nitro+ 8gb from 1240mhz to 1150 (trying to use less electricity, i lose i hole Mh/s going from 32.2 to 31.2
you can try it yourself. open Polaris bios editor, open your own bios mod, and try the "one click patch". That will apply performance timings for elpida. maybe that would do the job.