I'll answer the second part first - Elite strikers generally overachieve their xG but it's by maybe 2-3 goals a season mostly because these strikers are elite as they manage to find themselves in elite positions. For example - over the last 6 seasons,
Ron has scored 8 goals more than what was expected off him (and that's primarily because of the goals scored via his head; if you remove the contribution from his headers, he's actually underachieving by 0.6 goals over 6 seasons),
Aguero 6.5.
Kane (overachieving by 22 goals) and
Lewandowski (underachieving by 7 goals) are outliers here and that's primarily because of 1 great/poor season.
Messi is another exception who has overachieved the xG (by 35 goals) but that's more because he's so good in scoring from outside the 18 yard box (where xG assigns a lower value to the shot) and he's been overachieving those shots by 17.4 goals.
Now about
Werner - he's overachieving this season by close to 3 goals - nothing extraordinary. But the question is if he'd find himself these sort of opportunities because PL teams just don't play ridiculously high defensive lines which helps players that possess lots of pace. In terms of finishing ability, I'd say that would remain roughly similar - as in if he's overachieving his xG by say 10%, it'd remain close to 10%. As an analog, I'll probably bring up the case of
Auba - He's performed close to +/- 3 goals every season for the last 6 seasons except this one. However, what's interesting to note in his case is the xG90 column which has gone down since his move to Arsenal by close to 25%, and that's what I'd expect to happen in Werner's case as well especially as I'd expect similar levels of service at both Chelsea and Leipzig (Leipzig have 70 xG in 30 games, while Chelsea has 56xG in 29 games - basically a 20% difference in xG here which is roughly the difference in Arsenal's and BVB's xG difference in the season Auba joined)