I just rewatched the sprint several times (on a very crappy youtube upload) and I don't know how you can say Sagan was the fastest. I'm not sure where Sagan was around the curve, but he was two bike lengths behind Cav since at least ~150 metres from the finish and didn't manage to make up more than one bike length, despite having Goss' and then Cav's wheel and despite Cav already having been in the sprint several hundred metres. He just didn't have the legs.
And awesome pull by Wiggins, wasn't just for show, he gave it all he had.