The Sion Mana cost nerfs on the PBE are pretty fair imo
Decimating Smash (Q) mana cost reduced to 40/45/50/55/60 from 45/50/55/60/65 Soul Furance (W) mana cost lowered to 60/65/70/75/80 from 65/70/75/80/85 Roar of the Slayer (E) mana cost increased to 35/40/45/50/55 from 35 at all ranks
Basically, Q and W collectively 10 less Mana and E is 20 more at Rank 5
This was done no doubt because his E is maxed first and mostly just really low cost poke and they want people to pay slightly more for constant poking without trading