There are 2 sides of the coin:
Hecarim is solo top or jungle champion
Jungle he needs Q to clear faster, and i really would not make him overpowered.
Lane needs Q to harass
Everytime you harass youpush your lane
Do you really wanna do 200 damage to each minion to get some damage onto the enemy champion? Get the minions under his turret, where you can be easily ganked, is hard for you to farm?
Riot is trying to balance this. Putting it to an amount where jungle clearing is very good and wont push lane too much.
Amumu tears deals %HP, minions % is low, so not much pushing. Shyvana deals i a VERY small area, and it is damage over time, meaning you can simply juts walk away from minions dealing just a little bit of damage, instead of 200. And mundo burning agony can be deactivated at any time.