With regards to Werner's numbers, one needs to be wary of players from teams like Leipzig, where the whole footballing project is so structured and organized to be very efficient. Most of the players and staff in the club will have been handpicked to fill specific roles, which may not translate very well in a broader sense. I think this has been true of a lot of teams in Germany where they don't have the most money so have to make full use of the resources they do have such as scouting, youth development, and coaching.
In any case, he's a good player who has already undergone a good development, so I won't be surprised to see him do well, provided Chelsea has the same "more than sum of it's parts" mindset. If not, he could very well struggle with the league winning expectations. The PL has recently been more of a "money-splashing" league than a developmental league, so there can be a lot of adaptation involved for some players. Although managers like Klopp and Wilder are changing that slowly.