The old bowls need more water to clear the bowl.
I've seen a new tank on an old bowl, and it just stirred the water, it didn't flush it.
Toto makes some very nice low-flow toilets with classic styles that would work very well in commercial usage.
Joerg, It's too bad about your situation.
I've installed plenty of the Glenwalls, from American Standard that worked fine.
It sounds like you got a defective bowl on your American Standard Yorkville.
I would have them replace it with a tested bowl from the factory.
I sell about 2000 new toilets a year, and if I had those kind of problems, I would find something better to do.
Thank goodness, that most of the customers use standard floor mount toilets, which tend to work much better than the rear outlet kind.
I also sell mainly Toto, which maintains excellent quality controll.
It's too bad they don't make a rear outlet toilet that would work in your situation too.