Allowing for easier headshots is besides the point. Focused fire is supposed to be a damage output buff, for any shot. It isn't... it's bugged.
(this isn't the only 'vacuum' study on this either)
Why would you claim that....? This test on reddit doesn't say anything of the sort. If there is another that indicates that, I will stand corrected.
It is the very first chart in the research you linked to.
Focused fire shots do more damage per shot no matter what enemy they hit or whether or not it is a precision shot.
He then calculates
damage per second by extrapolating from the time it takes to empty a magazine ([damage per shot] x magazine size / time to empty magazine), and then
sustained damage per second by also factoring in the time taken to reload.
DPS and sustained DPS are higher without focus fire solely because of the higher rate of fire without focused fire. If you hit every headshot firing from the hip (or without the perk selected), you will do more DPS (again remember per
second, not per shot)- the additional damage provided by focus fire isn't enough to compensate for the higher RoF under the conditions that every precision shot hit. This is accurate math. But in practice I can't
hit headshots with every bullet with a high RoF auto rifle. So of course allowing for easier headshots is relevant. It's the primary benefit of the perk.
I went through this exact same argument about the SUROS Regime which, you must understand, involves the exact same scenario (lower "idealized" DPS with focused fire than without).
Nothing is bugged with regard to the Abyss Defiant, the research you point to doesn't claim otherwise. I'll just reiterate what I originally said, if you do better with it without focused fire, more power to you. It makes it much easier for me to hit headshots and that's much more important to me.
just as an aside, I notice the author of this research is pretty taken with the impact of Hive Disruptor (I haven't unlocked it yet)