Could someone help me with a problem? I switched 2 RX 460 for 2 RX 570 and I gained per GPU ONLY 4 MH/s.

The two XFX RX 460 were 4GB only and the new two MSI RX 570 have 8gb and more bit-rate on the top of it. One RX 460 4gb was giving 10mh/s~ with a variation of 100h ( like 10.1mh/s and 9.9mh/s) and now one RX 570 8GB gives 14 MH/s with the same variation.
All GPUs in the rig: 2 R9 380x 8gb ( each gives 28mh/s, 40 computing units detected ) and 2 RX 570 8gb (each gives 14mh/s, 32 computing units detected). So the problem or so I believe is that the RX 570 is under performing for some reason and my commands doesn't have anything restraining performance, because RX 570 has far more computing units AND 4GB extra than one of the old gpu. Could the mix of old and new gpu affect the performance of RX series?

In short:
- switched 2 RX 460 4gb for 2 RX 570 8gb, gained only 8mh/s with that exchange meanwhile an old gpu like R9 380x gives way more mh/s.