As title suggests, the current stats of mercurial armour piece only provides increase movement speed low, which is matched by black kat armour. it would be more reasonable increase the movement speed to medium on each piece, providing more incentive for the armour that is designed to have superior speed.
This buff would not be game breaking, as there is currently no real reason to use mercurial armour due it's lacklustre stats.
Please consider this minor buff to make mercurial armours more viable as the best movement speed armour.
Yer looking at this from the wrong angle, BK should be nerfed. I would say BK needs to lose the MSI bonus.
[edit, err]
I'm not all too sure about increasing MSI on mercurial, it seems like it would be a little too powerful if coupled with Swift Steps II. I'm not too sure about that though.