The AMD card I'm currently using on XHV is a Sapphire Nitro+ 580 8GB.  Bought it used off of ebay, dont know if the guy before me has modded the BIOS, I havent looked at that yet.  Im using "cast_xmr-vega-win64_096" as the miner.  The miner recommends to use the Driver 18.3.4 and to set the Radeon settings to "compute" and then also use the "--forcecompute" argument in the command.   At that point I starting getting more than 800 H/s, it ranges as high at 1100 but Id say averages about 880 most of the time.  Only tweeking on the card I did was that I have MSI Afterburner OC'ing the memory to 2200 (core/ power/ everything else default).  As a side note, getting my system to use the 18.3.4 drivers has been damn tricky because the integrated R7 GPU in this A10 CPU wants 18.2.2 and that older driver kept over-writing the newly installed 18.3.4.  Finally had to just leave the integrated R7 GPU as a "basic microsoft display adapter" so it would stop Fing over the RX580 driver-wise.  I would have disabled the integrated R7 but I like leaving my HDMI cable on the integrated port and off of the card as in some of my rigs Ive noticed leaving an HDMI cable on a mining card periodically drags down your hash output.  Also once I forced that upgrade on the Radeon drivers then I could not open the Radeon settings on that point to switch to compute mode, kept getting an error that versions werent right.  So had to open the registry to force that compute mode.

Im starting to experiment with 2 1080TIs 11GBs.  Seem to get about 2KH/s total for both cards using "xmr-stak  2.4.5" as the miner.  It recommends running several "set GPU" commands right before running the miner and I have let it run for a few hours both running those commands first and then rebooting and doing it again without the commands.  So far for me the resultant hashes is about the same either way.

Hope that helps