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Try below on terminal before starting mining.

export GPU_FORCE_64BIT_PTR=0
export GPU_MAX_HEAP_SIZE=100
export GPU_USE_SYNC_OBJECTS=1
export GPU_SINGLE_ALLOC_PERCENT=100

And it may be helpful adding '--cl-local-work 256' option to the end of your ethminer options.

That helped a tiny bit. Up to 7.8MH/s on the RP560 and 15.2MH/s on the RX580... This is just weird. The only thing that changed was the software update.

I'm sorry to say that maybe Apple has to optimize device drivers more for 10.13.2

I have almost the same set up and ran into the same issue ☹️