I could be way off here, but my understanding is that people usually farm 2x4 because of the way spawn rates work in retail; you end up maximizing profit and loot per hour on exactly 8 accounts, which helps pay the extra 3 from an economic perspective (given that in retail you can fund accounts with gold). In classic, the spawn rates are just atrocious and there is very little incentive to go beyond the usual 5, both from a loot/xp perspective and from an investment perspective (being that you are literally paying 3 more accounts just to be a lot more inefficient). Turns out, besides the Shadowpriest mechanics, you can achieve most of your listed synergies with 5 accounts.