You can try to use the alternative kernels (-clkernel 2) to see if this will solve the problem.
-clkernel 2 = 13.157 MH/s
-clkernel 1 = 10.856 MH/s
-clkernel 0 = 8.858 MH/s
Not bad at all, but still lower than Claymore.
We will try to source a gfx804 based RX550 to see what is going on.
I'll be glad to help. Let me know if you need more logs, test new build or something else.