This build focuses almost entirely on flat AP, since Sion's AP ratios on both relevant spells are 1:1. If you want a bit of change, swap the Deathfire's out for a Nashor's (ult synergy) or even a Guinsoo's, if that's your thing. Try to keep Golem buff on yourself as much as possible, the CDR and mana regen will net you more than a few kills! Skip Mejai's if the enemy team doesn't look like they'll allow you the kills!
--- This build is still in test mode ---
I'll update it when I've fully tested it, but as of yet it's looking pretty solid. The AP provides you with more shield-survivability and insane nuking damage. Go out and crush some boneheads with it!