FBB:
BS > Twin > Demo > BS > True > Snap > BS > True > Snap > BS > Twin > Demo > etc.
R > F > R > R > R > F > R > R > F > R > F > R
FBFB:
DK > Twin > Demo > BS > True > Snap > DK > Twin > Snap > BS > True > Demo > etc.
F > F > R > R > R > F > F > F > F > R > R > R
That's quite a lot more movement from flank to side than the standard FBFB, unless I'm completely misunderstanding what you're referring to as FBB... because the above shows 8 transitions in FBB versus 3 transitions in FBFB. Not to mention that FBFB keeps ToD in your comparison, which can often help with the target moving since it has no positional dependency.
Then, you say it yourself. This whole thing, even if pulled off, leads to 3-5% more DPS? Oh man, totally worth stacking MNKs on everything! A 3-5% increase falls in the realm of standard deviation among how much DPS a MNK will do in any given content depending on RNG.