I see what you're saying, and yes a phantom dancer could be used, but i believe the aoe damage from the tiamat will help quite a bit more endgame. My reasoning behind this is simple, pop ult come uninvis from behind. ALL will be hit. when they all go to run, expunge. Penta kill.
Edit: My name on LoL is the same as on here, add me and i can show you how my build puts twitch to his full potential.

I like the build, but i would add a phantom dancer as 4th item rather than a tiamat. The tiamat isn't really beneficial enough. The phantom dancer adds attackspeed crit and movement making twitch lane faster and kill faster. Please reply with an explanation for the tiamat.