If you really want to buff Sion, make his W have a lower mana cost. Having enough damage, even on W, has never been his problem. His problem is that he can't cast his spells as often as he would like to because of the high costs, especially for a champion that doesn't have many options for mana aside from Sheen items or Frozen Heart. (lots of other mana intensive tanks typically have the option to buy RoA and have the ap benefit every ability, such as Nautilus or Maokai)
It seems like the buff is geared towards his lategame, so just make his W's mana cost not increase per rank. Voila, Sion buffed without making his W damage stronger than it needs to be.
If the goal was to make Sion's W more compelling, give it more health scaling, like on the damage. The overall damage doesn't have to go up necessarily, it's just that scaling with his health feeds into his core gameplay of
1. Kill minions
2. Stack health
It makes farming and scaling up with W's passive more exciting, even if he isn't actually stronger than before.
It would also feel more exciting if his shield amount scaled with bonus health instead of total health. The shield in relation to your total health never really changes, which keeps it from feeling as compelling. The shield becoming larger in relation to your total health as you get beefier would make it feel more exciting, at least visually. (This would be a bigger balance change though that might get pretty funky, I haven't done the math or anything.)