So I finally got the 5th class rank 2 days ago, and I became a Corsair.
I did some tests with the 1h sword + dagger vs 2h sword, using both Z and C to attack make it too slow, and if you use only Z or only C the damage is a lot lower than using 2h sword, and I did the tests with lvl 120 1h sword + dagger vs lvl 75 2h sword.
It just makes no sense for a dual wielding class to do better with a 2h sword than with the proper 1h sword + dagger.
