TL;DR version
Either A) give the kat hiss hood and mercurial demo helm medium speed boost or B) give the kat hiss hood and the mercurial demo set medium movement speed while charging bombs per piece.
Currently the kat hiss hood is the least popular of all the kat hoods, and for good reason. It isn't terrible but you can't work it into any current bomber sets because it is shadow defense. I feel the best way to balance it out with the other hoods to at least make it appealing is to add some speed boost to it. Maybe even make the speed boost only on bombs to avoid players just getting it to move faster so they have to be holding a bomb to see the effect.
This buff should also be applied to the mercurial demo helm instead of being a lesser, but a variation so either one will work well.
Or to take this to a different direction, we could instead make movement speed buff simply cause the player to move faster while charging, this way we can buff it to medium on the whole mercurial demo set and then restrict it to bombs only.
EDIT
So to keep the MS (quick silver) set to not be over shadowed by the MD (mercurial demo) set I feel the MS set should have medium movement speed per piece. Maybe even get a SL armor seeing how it is a boss set...?
WHAT DO YOU THINK?
Errrr....good intention, but what about Mercurial? (The quicksilver one) This would be left completely in the dust, IMO. I still say that they need to balance MSI, and then give each 5* Merc Slime (what I'll call it) a High MSI.