Hi,
i have small rig with 6 Aus Strix RX470 with moded bios.
In last time i saw that speed is lower, i check some logs and:

01.06.2017: 164MH/s
10.06.2017: 164MH/s
20.06.2017: 162MH/s
01.07.2017: 162MH/s
09.07.2017: 160MH/s
16.07.2017: 159MH/s
25.07.2017: 158MH/s
30.07.2017: 156MH/s
03.08.2017: 153MH/s

and rig is working all the time without any other problems. I use Claymore 9.1
but i try also older and newest version. For now i use Clay 9.8 and speed is 153MH/s (~25.6 per GPU but 2 month ago was ~27.3).

I was on hollidays ... so What i missed ?
Thats normal ?