I am seeing the same thing on my RX580 rig with 6 cards.
Was getting a very stable 28.5 MH/s with mem clock at 2020 MHz.  No memory errors in HWINFO64.
Any higher in the memory clock, errors start to pile up fast.

But now I'm testing at 2100 MHz clock, getting very close to 31 MH/s now... 
Reported and effective hashrate is way up, 0 invalid shares, no change in stale shares.

Memory errors are through the roof.   I'm talking in the billions /day.   Doesn't seem to matter, I'm seeing an improvement in profitability.
I hope it doesn't harm the cards.