The 50% was a compensation for the removal of GW, I like the idea of insta strut but with it being knocked off by a minion it makes it rather useless. It can't be used much for chasing because again minions, and also kiting by the enemy adc will knock off the movespeed. If it could not be removed for the duration of impure shots it would help out with what you are doing to her W.
20% is not going to make much of a difference, but 50% will make enemies a little more cautious on how long they stand off with her. With more attack speed she can utilize her W passive to better effect for that magic damage will start to ramp up.