Please help: 2 of the 4 290X are way too slow when performing dictionary attack

I will make it as short as possible although there are many background details:

I have 4 R9 290X in total.
2 x "Asus Direct CU2 R9 290X OC" are directly connected on a MSI Z87 MB and 2 x "Sapphire ATI R9 290X" are connected via USB 1x to 16x USB risers on the same MB.

The 2 Asus, MB and Hard Disks power up via a 1100W PSU and the 2 Sapphire via a 850W PSU.

When trying pure brute force with oclhashcat (MD5: -m 0 for example), all 4 cards show/have more or less the same capabilities in terms of computing power.
On the other hand, whenever I try to do a dictionary attack (see attached where -m 1600 (Apache MD5 APR) is used), the Sapphire cards always drop at approximately to 1/3 of the Asus computing power.

I have tried numerous things to no avail. Results are always the same.
Any help will be greatly appreciated.


Note: on the attached photo GPU #1 and #2 are obviously the Asus and #2 and #3 are the Sapphire.




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