I play wow almost exclusively.
I have a r5 1600 oc 3.8 system with rx580 8gb, and also put together a r6 2600 (stock) with rx570 8gb for a real-life guildie. ( I played his system for two weeks before giving it to him).
seriously I feel no differences between the two system when in mythic +/ raid. keep in mind your fps will dip down in the 30-50s when your are dealing with open world boss like the one in arathi and dark shore.
overall fps is around 80-100 in Zuldazar pyramid, and usually 150+ during flight.
as for your overall build, I can tell you can getting a 2600 (stock fan) and rx570 makes no noticeable differences in wow performance vs a 2600x ( /w gammaxx 400) and rx580. however the former saves you about 60-80 dollars. if you do intend to play other AAA games in the future, you could do an upgrade on rx570.
one thing I like about ryzen is that even with a budget build ($750 USD) you can open OBS and stream your game session (sometimes you need a 6th person to guide you with a mythic or something), your gaming experience is still quite enjoyable with OBS on.
if wow is all you play and you want to have maximum performance, I would suggest you to get intel system.
oc any of the k sku coffeelake (8350k,8600k/9600k 8700k/8086k/9700k/9900k)over 4.8 will net you pretty significant fps gain over the ryzen system (but quite pricy)
if you want to know the relative performance of high systems ($1500+), my other frd put together a 9600k (4.8ghz) /3600 ram / 2080 for the upcoming 8.1.5. under 4k he uses 7 for non raid (120-170 fps), and 5 for raid/dungeon (roughly 150+ fps). In terms of monitor choice, 4k60hz give you very crisp image, and 2k 144 give you very smooth gaming experience. (give you have a strong system)