Cho'Gath main here (admittedly could use some work on my play). I honestly think he's fairly balanced. If you play well, against most meta top laners, you're going to be okay, and against some of the squishier ones, you're just going to outright win. His passive is fantastic for sustain, allowing him to tank a lot more damage than you would think, assuming you CS. Assuming you've leveled your silence or have landed a knockup and auto-attack with leveled Es, you have a lot of damage.
See a trend? He rewards playing him well. He's balanced around you being able to utilize his passive and use his damage.
I would say he isn't perfect, though. He's slow. Painfully slow. If you're chasing and haven't built movespeed beyond boots, you're likely going to be outrun by most champions and not be able to land a Q. And if you DO land a Q, you often can't move fast enough to body block or silence for your team to follow up. That in mind, he only really works in teams that already have hard initiation (eg Leona). He has fantastic follow up, since the q is very easy to land against enemies already under other people's initiation, but compared to other champions that work reasonably on EVERY team composition, he can be a bit underclassed towards the late game.
If he needs anything at all, I'd say he needs movespeed. A flat movespeed buff might just be overpowered, but maybe a light boost on his Q.