The odds for the German visitor attraction have shortened from 6-1 to 6-4, making it the bookies' top choice.
Perhaps surprisingly, the Richard Rogers Partnership's Barajas Airport has slid from 7-2 out to 9-2.
The Brick House by Caruso St John, a definite dark horse, has held steady and is now a solid second favourite at 9-4.
And the odds have also shortened for the Welsh Assembly, also by Richard Rogers, from 9-2 down to 3-1.
The Idea Store, by David Adjaye, which was joint outsider with the Phaeno when the odds were first released, now holds the dubious honour on its own.
Changes to William Hill's odds are likely to have been influenced by a series of 3-Minute Wonderprogrammes, shown on Channel 4 this week, that detail the shortlist.
The winner of the Stirling Prize will be announced live on Channel 4 on 14 October.