Eh. Still not a fan. If it wasn't a +3 effect... maybe. But that's a pretty nichey ability that doesn't scale well-- starts early, if your game goes past level 12-13 it falls off hard.
Although a Kensai can easily take the whip, sawtooth saber, or the dueling sword, so slashing grace is usable by a kensai without dipping swashbuckler.
Weapon finesse doesn't work with sawtooth sabres, so you do need a dip into swashbuckler if you want dex for both to hit and damage. The sawtooth sabre is considered a light weapon only as regards the two-weapon fighting penalties.