Is a Shimano Dura-Ace 7800 rear derailleur compatible with a Dura-Ace 7900 11-27 cassette?
The RD-7800 manual states that the largest sprocket for a SS is 27T.

Manual here https://static.bike-components.de/cache/dl-rd-7800-2-pdf-ce95c6782564d2bbf75cca831ae0bcbf.pdf