Meh, Hexakill on SR doesn't differ that much than on TT. It's a very slight modification to the meta. an extra support or an extra Jungler.
On TT, Their's no room to really think about a meta. (though some people could imagine one.) Probably part of the reason why everyone thinks Assassins are the core of that mode. And even though they could work. If you end up with a team of 6 assassins, get ready for a kick in the balls.
People forget that Assassins are single target, Even if you where playing a clean up assassin like Kat or Yi, you will likely get crunched by someone who can do a lot of AoE damage (Cause TT is fairly close quarters) Personally my goto champ is Karthus. Because a) he does a lot of AoE damage, and has a lot of team fight tools. b) because of the high death nature of the game mode, his passive gets a lot of mileage, and c) because he can patiently stack a tear and a RoA, I find him less CS reliant, than some other champs)