There's definitely something wrong with your setup. You should be getting about 24 at stock. EDIT: or maybe 22 since they are 4gb
I really do not think so, I have 6 Asus Rog Rx 470 4G hynix, 1 Gygabyte Rx 470 4Gb gaming 1 hynix, and 2 XFX Rx 470 4Gb samsung dual BIOS.
Except for the XFX (Which seemed to be already overclocked), all of them hits 20 at stock, I tried claymore 9.5, 9.7 and windows and linux.
Overclocked all cards gets 24,5 in windows (linux stays 20)