It seems like rx560 is quite good in performing cryptonote algorithm compared to other gpu with same pricing range.
I think the RX 550 is also quite good in performing XMR ... more than 400 h/s with less power consumption compared to the RX 560.
Are you sure you can get more that 400h/s on 550? It seems it's also difficult for 560 to hit 400h/s. For a 550 it looks like a miracle.
The RX550 is the perfect gpu for mining xmr, because cryptonight charges the memory and 8 gpu cores are enough for the rest. The bottleneck is the memory.
What brand is your rx 550?