Yes, it's 10/20/30/40% damage. Also don't forget that Greased Lightning's speed bonus (5/10/15/20%) isn't just to weaponskill recast, but also autoattack speed.

The reason monks have this bonus is because their weaponskill potencies are preeeeeetty crap compared to most jobs, so they need these big multipliers to make the job worthwhile.

And no damage buff from any job is negligible. Jinpu is only 13% compared to the 40% from Greased Lightning, but no samurai in their right mind would ever say "nah 13% damage aint my style."