I like seeing the bonus at T4 instead of T5, but why restrict occult slayer to med/hvy (the worst & weakest kind of melee armor). It's already the least used Barb Tree. It also nerfs existing light & cloth slayer builds, basically requiring them to drop T5 slayer for FB or Ravager.
If it's just to compensate AP spend balance for a T4 feature, then OS should have a T5 25% as well without armor requirements.