did you flashed bios of the cards or overclock only? And which brand of rx550 you use?
I had similar result as the other guy... Most of my 550s are up 1.5 to 2.5 MH/s...
Mine are all ASUS RX 550 4GB. All have a bios flash. Most cards are 1250 gpu, 2000 memory. However I have a couple that I can't get above 1850 memory. I have 18 of these...