That's true, once rupture lands, it is very easy for the Cho'Gath to follow up with his combo, but what I find about Cho'Gath is that he tends to be weak later games for some random odd reason, his damage doesn't scale very well late game other than his ulti o.O And if he build mage then he's a reasonably easy target, if he build tank, well, you can just ignore him majority of the time after he used his ulti. One thing I tend to do is that if I see a Cho'Gath, I avoid engage in jungle, like you already mentioned, his rupture is very very good in jungle due to its area that it covers, nonetheless, there are hardly any good Cho'Gath players that I seen recently. If you're against a mid Chogath you should have no problem farming, it is top lane that is annoying, due to majority of the top lane is melee, making dodging rupture + farming a lot harder than any ranged mid. Personally I love seeing a Cho'Gath when I'm Elise :P Since my Q always scales hp, with the more rupture feast stacks he have, the more damage I do him, huehuehuehue, the new Elise is so awesome after the buff :3
Will head out for lunch, be back later guys :3