Yes and no, the amount of farm it takes to get to that point is way above average. To really maximize AP Sion, you need Lich Bane on top of a Deathcap, plus boots and a couple Doran's Rings. Yeah, you do get a good laning phase and some good ganks, but that's what you really need to be able to kill a carry late game -- plus a Banshee Veil pretty much ruins you. I'm not looking to really make AP Sion better, I'm looking to make Sion fit his own character while still being equally viable built AP -- he's like Morde or Singed flavorwise, but plays a bit like a watered-down Annie --- the stun being more reliable is nice, but it's considerably less of a big deal now that it has been nerfed.
He used to be classified as a tank and a mage, now he's just classified as a Fighter, but he doesn't fit the Fighter role that well at all -- he's not tanky enough without sacrificing damage, basically, the ideal build would be an AP/AD hybrid so you could do a decent burst and then use your ultimate to leech doing decent DPS until your stun was ready again -- which would fit the unstoppable juggernaut image portrayed in his flavor text.
But to accomplish that, you need too many stats, and it's just too much money to get a character that doesn't really equal his rivals. Jax, Morde, Singed, yeah, pure AP he does more burst (maybe) but his sustained is worse and his tanking is worse. He's such an early game champ in his actual gameplay yet the way his "E" scales suggests he was intended to scale better into the late game than he actually does.
If he's to be even with his main rivals, he basically just needs some attack speed and some other buffs so that his sustained presence and tanking are a little better while not hurting his initial burst too much.
He used to be an awesome champ (I mained AP Sion for a long time, and played a fair bit of AD Sion too) but now the only reason he's good is all the experience I have under my belt. Hextech Gunblade, for example, should be an awesome item on him, but it's not, and a big part of that is how slow his attack speed is and how badly it scales. Morde, for example, at level 18 has >1.0 attack speed, Sion end-game has barely .9, might even be in the high .8. You go to use his ulti, which adds 50% attack speed, and you're still not faster than some characters' base level 18 AS! Dumb. Even built AP, his ulti should be more meaningful in a team fight, built AD, he shouldn't require more items than every other AD melee to compete.
I mean, if you want to keep him exactly the same, I guess you can, but then he's just a less viable version of other mages who doesn't need to build RoA to have good hit points, but whose damage is less. One Sion combo is less than one Annie combo, Brand combo, etc., unless he has Lich Bane which requires him to be up and personal with the enemy -- I'll say one thing, his combo is better than other mages when those mages ultis are down, but Annie, Brand, etc. can also repeat their combo faster and chain spells together more efficiently.
Sion's shield could be really awesome if it burst when it took enough damage, making him kind of a suicide bomber, , which if Riot were to do a rework, would be my #1 wish -- but it doesn't, it just fizzles out -- it would be very cool to tank him out a little, get a Deathcap, and start beating on the carry with autos with everyone afraid to attack you cuz they might accidentally kill the carry from bursting your shield, all the while throwing down stuns left and right to really disrupt a team. Then your shield bursts and you're pretty much dead, but for those 10 seconds, you'd feel like the unstoppable berserker you were born to be.
Sion just needs some attention, there are any number of ways to do it -- I just hope he doesn't end up reworked and turn out as lame as the Kayle or Tryn remakes. Wouldn't mind if it turned out to be more of a GP remake, though.