That kind of theorycrafting at endgame is HUGELY reliant on;
1> Tiny little statistical differences; one AR having a SLIGHTLY higher fire rate than another makes that other "useless" in their minds
2> Having the right set bonuses for the gameplay style you're aiming at; some sets favor ARs, others favor LMGs, others marksman rifles. However you want to play, there's a set that'll cover you, basically. My gear's nowhere near max, but as a solo player, the Nomad set's pretty solid (no weapon specifics, gives you constant health regen and health-on-kill healing). I may only want the 3-set bonus, though; the 4th is a free revive every 4 minutes, and if I'm not playing stupidly, I shouldn't NEED it, so getting a set bonus that helps me NOT get downed seems more useful.
Youtubers are often in groups, too, so they have the benefit of hosing down dudes who are aiming at their buddies, not themselves.As for tactics, I sort of gathered as much. Attempting to do something a well geared Youtuber could do (burst down normal enemies outside cover) doesn't work too well with leveling gear. If I have to hump cover to regen health between shooting at enemies, then hump cover I shall!