Ok. Let's call it success

I've flashed two my cards and run a heterogeneous rig with 3x R9 390 and 2x RX 480 (flashed)

I called it "success" because after flashing each card individually with fresh driver and signed atikmdag.sys gave me 29.2 MHs

After adding all 5 cards in rig they can do exactly as they did before - 24.5 MHs

All is with CDM and single mining ETC

I'm sure if you will have only RX 480 - they will work fine, but in combination with other cards (in my case R9 390) I've got zero gain