Currently it is bugged.
In regular form, - sneak attacks damage does not multiply on critical hit.
If an instant attack criticals, will calculate the full damage of the sneak then multiply.
I have confirmed this on live server with a deep-rogue weaponmaster.
Regular sneak - 70-80 damage
sneak critical - 110-120 dmg
sneak whirlwind critical 180+ dmg
10-20 crit range whirlwind attacks for x3 doing nearly 200 dmg is absurd on a dex build.
