This is why 10% to 20% here or there means nothing for me in reality - its not going to magically fix the problems in the game.
Also,I do know a lot of WoW players(even though its not my cup of tea),and only one of them has an overclocked CPU(an FX one BTW) and none of them have a card over £300(since most are not really into PC hardware that much),so it probably is running even worse for the average player.
Luckily,it has a cartoony art-style so you can drop settings without much of a visual difference. Interestingly one of my mates mentioned some of the add-ons are very poorly optimised and he just changed to another one and it actually had a decent performance boost.
I have a member in my guild that plays WoW and raids at Mythic level with us on a Mac Mini using an Intel IGP of all things. When the performance craters for us; it downright crashes his little machine.
Other's still use GTX 660's, and one recently upgraded from an Athlon X3 to a 6600K; which was a massive improvement. Then again that would be a massive improvement in every computing scenario possible.
It's true the game is extremely scalable, and you can also still switch between DX9 and DX11 depending on hardware to find the best balance of performance.
I can vouch for your friend and WoW-Addons. Some just destroy performance; many of them are logging ones though. For recording Damage/Healing Per Second to determine each players optimal performance in scenarios.
Some even had horrible memory leaks that would use up all 32GB of my system memory.
Even then they all do more than play WoW, so their system needs to be used in many cases. Hence the more techy ones not going full in on pure Single Threaded performance.
It's why I'll always recommend people look at their needs and use cases to see what's better overall for them.
I could have gone 4790K at the time and overclocked it to the moon, instead of my 5820K, but it wouldn't have done as well for my other needs. The same will be the case for many between the 7700K, or 1700/1800X if I had to make that decision.
Then factor in pricing between Z270/X99 and AM4, and that more and more future games are becoming more Multi-Threaded and I know where I'd lean to, especially if RyZen has comparable IPC to Broadwell-E, and comes with better base/boost clocks from the start.
^^^
Strikes me that if you are a WOW fan, you should disable all but two cores and start your system from there with a massive overclock and cooling to suit. However to decide the CPU market requirements on such a user requirement is a bit narrow. Maybe an overclocked i3 is the correct tool for the job.