I don't necessarily see a huge significant increase in crits, but, it does only last tree turns. I think ... well, I think that it should last for 5 turns, and have Grand Shadowdance last for 7 or 10 turns.
I think that would REALLY make us swashbucklers notice if there is a good increase in criticals or not.
Especially since I have no clue what affects crit, or what the default crit is, or what raises it or lowers it, etc.