Generally, 470's will get about 570-580 un-modded and around 690 after that strap-mod.
I have 1 rig in each case, and that is almost exactly what my 470's are getting under both scenarios. Granted, 480's should probably do a bit better than that but not my much.
Are you sure about that?
RX480's : GPU0 788 h/s, GPU1 783 h/s, GPU2 791 h/s, GPU3 786 h/s, GPU4 789 h/s
That's around 100 h/s more than your 470's.
If you can confirm what bios gpu,memory straps are you using, it would be great. and also are yours 4gb or 8?