Increase their model count to 60. Decrease their ammo count to 12.
20 * 45 = 900 ammo
12 * 60 = 720 ammo (drop of 180 shots)
Justification is that they cost 1100g and are supposed to be strong melee combatants, they have less ammo and range than WW, they have low armor and they are not AP. Poison is only valued at 100g as shown by other poison archer picks.
They are never going to impress in melee at 45 models, which is the unit size of elite cavalry skirmishers that cost far less. Also, at 900g Shadow Warriors out shoot and out melee the higher price tag Shadow Walkers.
Lothern Sea Guard are 68 models for example and cost much less.
Alternatively Shadow Walkers could stay at 45 models and reduce their price tag from 1100 to 900 which is 225g more than what a 45 model unit of Shadow Warriors would cost. This price might still be over priced.
900 (SWarriors) * 45/60 = 675g