"Schon jeder zweite Berliner ist unser Kunde,"
Translation: "Every second Berliner is already our customer,"
Does that sentence emphasized the wrong word?
Should it be: "Jeder zweite Berliner ist schon unser Kunde," to mean "Every second Berliner is already our customer,"
Both sentences are correct. "Schon" (=already) is placed at the beginning to emphasize that the target was met very fastly.
This is obviously from some commercial, where the schon is placed in front to emphasize how fast they're growing and how great they are, and shouldn't you join the fun if already half your town is buying at their place. Your second suggestion on the other hand is a neutral statement and isn't going to attract new customers.
The two sentences are not quite the same.
Jeder zweite Berliner ist schon unser Kunde talks about half of the people in Berlin, and says that they are already your customers.
Schon jeder zweite Berliner ist unser Kunde talks about your customers, and says that these include already half of the people in Berlin.
Or so they feel to me.
I disagree with your explanation, and I'm afraid it confuses people who are learning German as a foreign language.
Both sentences talk about customers in Berlin. And both sentences say that half of the inhabitants are customers. In both sentences the word "schon" implies that the number of customers is growing. Putting the word "schon" at the beginning lays more emphasis on the speed of growth.
To me - as a German native - both sentences express the same, that every 2nd Berliner is already your customer.
Both sentences (grammar) sound fine to me.
Sometimes you can move words between front<->end or somewhere in the middle before/after ..., and sometimes this may not work from the grammar side for a special word. You need to get a feeling for it.