The Impetus bonus adds increased critical damage per hit landed, and wears off the same way as the attack and critical rate bonus. This makes it by far the best body piece in the game for MNK when using Ascetic's Fury and Victory Smite. Probably still the best for Asuran Fists too, considering the accuracy, attack, and higher than Kirin's Osode STR and VIT.
Depends on enahnces Martial Arts effect really.
If Enhances Martial Arts is -10 delay, this body is probably behind Juogi+1 and Hachiryu body.
If its -20 delay, it pulls out ahead, but just barely.