This is a buff to sion's passive
Similar to the mastery "grasp of the undying", could sion steal a % of his max hp instead of getting lifesteal? He barely builds any ad when you play him right, and the lifesteal falls off hard. This is intended to buff his passive late game and keep him alive longer to punish those who don't focus him down in passive form, rather than just walking away, waiting for him to bleed out despite autoing champs.
That's my 5 cents.