Sion's real problem
Hi, I've been playing a lot of sion lately (mostly top), and even though I like his laning phase a lot, that's pretty much all he has: a good laning phase. Why is he not good out of lane? The reason is quite simple. He has nothing to stick to his target.
Now, before you tell me : "but he has 2 slows, and his ult makes him go super fast!" Yeah well that's the thing. First, his 2 slows makes him stop to cast them, which give the opponent just the time to get enough of a lead so they get far enough that sion still can't catch (not to mention the "E" is pretty easy to miss without minions and you probably won't have time to charge his "Q" for a knock-up). As for his ult, it's quite obvious it's made to initiate, and not chase (not to mention missing is quite easy and puts you even farther from your target).
In short, Sion need sticking power. Maybe give him some steroid (move speed), or a better slow, maybe some way to apply CC more easily, but he NEEDS something to stick to his target, because right now, any kind of slow, no matter how puny it is, will kite him to oblivion and make him close to useless.