As far as the few games I've played with Twitch, finding a Hard counter for him is quite difficult. I don't know about Higher ELO games, but thus far using 2-avarice build I've been able to roam quite freely while at the same time still have gold flowing into my coffers. And as far as EXP is concerned, a big wave of creeps+ SnP=1 to 2 lvls at a go.
So far, the only problem I've encountered is when the enemy shuts me down as soon as the 1st creep wave comes along. Be it having a champ that is nigh-impossible to harass or kill effectively with twitch and at the same time able to whoop his tiny rat @ss (I don't know any specifics, haven't met any but figured these kind of people will screw me up hard) and also tanks with CC(Cho , Taric , to a certain extent, Shen). Having said that, it is still necessary to hound him down with wards/oracles to prevent him from farming at all.
Still, at late game, unless the Twitch is seriously fed and has like 2.500 attack/second with about 600 crits at 50%,there are a lot of burst champs that can kill him fast enough due to his low health. Before playing Twitch, I used to play Tryndamere and from my experience, Tryn will wreck Twitch, if he is about equally farmed. The reason being Tryndamere has Spinning slash, which allows him to get close to Twitch as soon as Mr. T pops out, as well as Undying Rage which allows him 6 seconds of free reign from Twitch's burst damage while bashing Twitch with his own burst. If the Twitch waits for Tryn to die after rage, it is usually too late because the undying rage can wreck a whole enemy team if it's 5v4 without having to suffer heavy casualty to Tryn's team. Also, positioning is important against Twitch since his SnP hits in a straight line. If he can't get a good angle, chances are he will only be able to grab 1 or 2, at most 3 people with SnP, which reduces its effectiveness to a certain extent.
By no means are there hard counters to Twitch (not to such an extent as Oracle's is to Evelynn [If we consider this a "Hard" counter]). Just like all carries, he is weak earlier on and scales highly effectively. That is not to say that he is invulnerable or broken. I have played games as and against Twitches and there are times when Twitch dominates, and there are times where he doesn't. Ultimately, LoL is a Team Game(5v5 or 3v3) and no matter how good the Twitch player is, if he has f**knuts for a team then he is probably not gonna do much good.