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didn't he start as a time trialist?
He did. He was twice World Junior ITT champion, and finished third in the 2010 u23 World ITT, behind Taylor Phinney and Luke Durbridge, and beating the likes of Rohan Dennis and Tom Dumoulin. He turned pro based on his TT abilities, but then Skil-Shimano noticed his sprinting skills at a training camp and turned him into a sprinter. He won 17 times in his debut season, including a stage of the Vuelta.
His FTP regardless of weight is pretty elite at 430w.
I don't think his 3s power at 1940w, though really impressive is what his talents were at.
His average power (in the sampled sprints) for 15s was 1383w, with a max 15s sprint of 1602w.
I can do about 1000w for 15s (though 25kg less than Kittel) but probably couldn't even make it to the end of a grand tour stage, let alone 21 of them.