Thanks for all the input, but what I ended up doing instead of fiddling with the WattMan/Claymore settings, just to single out that 1 card behaving differently, was just flashing the ROM of all cards with the 480LowEnergyOK posted here:
https://bitcointalk.org/index.php?topic=1584617.0I am using:
6 x Sapphire Radeon Nitro+ OC 8GB GDDR5 (Samsung)
#SKU: 11260-01-20G
During the flashing I found that 1 of the cards was older and had a lower serial number than the rest, and it also had a different Flash Type, so the older card had a serial starting with A1651 and had a GD25Q41B Flash Type, while the other 5 had a serial starting with A1701 and had a Flash Type M25P20/c.
Before flashing, all cards were running 24MH+ with stock settings.
After flashing, the A1651 card is running 27MH+, while the A1701 cards are running 29MH+
There was also a difference in the Device Manager recognition of the cards after flashing, where the A1651 card is detected as “AMD Radeon (TM) RX 480”, while the A1701 cards are detected as “Radeon (TM) RX 480 Graphics”.
All cards are running the driver 21.19.137.514 dated 21.11.2016
The power consumption from the wall went from 1270W before flash, to 980W after flash.
Everything is now running stable at 173MH+ with no additional OC manually.