I don't actually see there being a good reason to run multiple desktop PC's anymore. For people that want to look at hardware broadcasting for better performance, I think the better option is to feed that input back into a single PC with virtualization. Sure you could feed that back into multiple PC's, but I don't see any benefit of doing it that way (outside of some edge cases where someone has 5 PCs already, so they don't want to pay for virtualization).

I started down the multiple-PCs road myself last week - hooked up the KM synchronizer to my desktop and 2 laptops for testing. It mostly works fine - I just couldn't get my Razer keypad to work with it (which won't be an issue for most people). I had cannibalized another older desktop some years ago, so I ordered parts for that and I was shopping for a 5th PC. Then I started looking at a better monitor setup...

In that time I started seeing what some others were doing with focus-follows-mouse and remapped keys for alt-tabbing. I think the performance there will be fine for 5-boxing- arguably better in some scenarios. I can program keys on my keypad to switch the current window of focus to which ever window I need, then to send a single keypress to that window - that all happens with just 1 keypress. So then you program multiple keys that you have multiple sets of those keys for each client. In combat you can send commands as quickly as you can roll your fingers across those keys. I need to do some more testing, but I think you can basically just mash all 4 or 5 keys at one time and each keystroke still goes where it needs to. Because I'm grouping my attacks onto multiple keys, I can't keymap everything like I used to, but I think I can get the 3 or 4 most hit buttons to all have their own group. For non-combat actions like drinking, mounting, etc (stuff where timing isn't critical), I'll handle those with a mouseover-focus.

These sorts of rapidly switching focus options seem to be the best footing for whatever blizzard decides to do in the future also - if they say, "no broadcasting by any means", I'll already be compliant.

More testing is needed to see how well this all actually performs in game - I've just done some very basic testing with multiple windows and notepad, etc - all seems very promising. The part that's kicking my butt right now is getting my screen layout optimized - but its slowly coming together.
https://gyazo.com/a4ad7d2987fd1632af9b94913789b900