The offset is usually for edge diffraction effects of the baffle. Basically a centered tweeter will see edge diffraction at the same time from each side...this can cause a drop in energy at a specific set of frequencies, as the diffrated energy flows into the room and interacts with primary sound fields. (Actual time differences with respect to the wooofer caused by three or four inches of tweeter movement in a perpendicaular plane to the listener are negligiable unless you sit in a close nearfield position with speakers placed far apart, in which case you should toe-in the speakers to get correct time alignment)
Therefore is does not matter much. Most manufacturers suggest to place the tweeters closer to the inside (this way the edge diffraction is least obtrusive as it affects the room side wall reflected energy most). The ear fails to distinguish directionality of edge diffraction from the primary signal when the edge is very close to the driver itself (like in a very narrow speaker)...it only begins to become apparent on quite large speakers (a foot or more wide)...furthermore edge diffratcion from two equidistant edges may alert the listener to a driver location - creating certain sounds that can be claustrophobic/locked or trapped around the speaker rather than flaooting free in the soundstage.
I have no direct experience with Sonus Faber, but for example, I have seen ATC speakers in many different configurations; David Gilmour has the tweeters closer to the outside edge in his Astoria studio, Abbey Road has them on the oustide too. Whilst the BBC has them setup with the tweeters closer to the inside.
In the end, the driver spacing between left and right channel and the distance to side walls becomes the most important factor to consider. In a small room with close side walls, then place the tweeters on the inside. In a large room it doesn't matter and comes down to preference.
Of course, soffit mounting eliminates this problem altogether and a well designed grill or rounded speakers corners can help a lot too. Some designs go for triangular and other odd shaped or angled baffles, like the Avalon line of speakers...in the end it is not a "big deal" whether the tweeter/mid's are closer in or out. So don't sweat about it.
