A multiplier of 0.92 (92%) is mathematically the same thing as an 8% damage output reduction. It's based off of comparing the tanks in defensive stance vs. being stanceless, purely for the sake of making a tooltip comparison between the three. Once you start trying to compare offensive vs. defensive stances in addition to cooldowns, you're better off just getting results from a target dummy, because the results will depend on how your rotation/procs/mp play out. Also, given that Sword Oath's dps increase is dependent on weapon delay, I'm not sure how you'd get a good representation of its contribution across all gearsets, unless there's only a minimal variation in the stat across all possible weapon choices.

On a side note, both A8N and Sephirot Ex have phases of the fight with purely magical tankbusters and purely physical tankbusters. In addition, A8N seems to encourage collaboration on these (read: likely to be enforced on a higher difficulty setting), either by timed swaps in the first part, or by a stack and split approach in the last part. What advantage you gain in one phase, you lose in another. I wonder if this is part of a broader design decision to balance out the tanks.