You understimate the power of 100% sleep (sleep granade / sleep entrance or whatever).
If you sleep a monster, for a 100%, even if it takes 100 tu - you take him out of the match for 300 tu. Thats alot fo time. Dont even dreamhunt him - just let him sit there for 300 tu doing nothing. Thats worse then getting killed. If the enemy sleep 4 of your monsters and get you into a sleep lock - match is over if you dont have bane / moni or whatevwr that counter it next.
So no i dont think they need a buff, sleep is the most OP thing in the game and need to be dealt carefully (buffs nerfs etc).