ETC is not using ethash anymore, while they are still somewhat similar, there are some notable changes, so you will need to work on it to fix the stale shares and lower hashrate, my recommendation is to get back to default speed and then work you way from there. I myself have a rx 470 and changing it from eth to etc had no changes in hashrate or stale shares but my card has minimal overclock.
Well there it is. Something stupid I overlooked. I'll start there and dig in.
I use ethermine (or whatever it's called now) for my pool and it was detected just giving all bad shares.
I run bios modded and overclocked (fairly aggressively though as I pay a flat fee for power) on mostly rx570s with a few stock bios rx-580s and one rx-480.
Thanks Metroid I will dig in more.