Considering that my swash is at 61 dps rating duel wield, 33 from items and another 28 from ring proc, the returns are already starting to diminish. Unbuffed (being the best possible scenario for sta) you'd be lucky to see 20% more base weapon dmg with sta. This is compared to the wis line's 68% bonus. The major reason wis was so viable back in the day was because you couldn't get double attack any other way and it stacked with everything else.
So, in short, nope STA line isn't a viable "dps" spec, even with the mythical.

... Oh here's bright idea to make wis viable again ... reinstate procs on double attacks while fencing. IE: +68% to procs, hurricane, inspiration, poison ect.