The following from Hannes Wahry of the Leica Shop in Wien, Austria appears to suggest there should be no noticable focus shift issues with current f1.5 50mm Sonnar lenses, unlike those with lenses adjusted to correct focus at f1.5 or f2.8

"Dear Mr. Bell,
the Zeiss info means (know it after a long call): The lens is adjusted at F 2.0 so it works to 99% perfect without front or backfocus. If there is any problem with your lens we have to send it together with your camera to Zeiss for adjustment."