My two RX550 runs stabel @ 13,8-14,2 MH/s on ETH. Cryptonight also about 440-460 H/s. Bios is modded.
What is your model/memory brand of theses cards ? I found gigabyte and xfx brands here but I don't know what will be better to do this mod.
I get the same results with my 18 ASUS RX 550 4GB cards, but I got all my card for less than $100.