Is a Shimano Dura-Ace 7800 rear derailleur compatible with a Dura-Ace 7900 11-27 cassette? Is the Shimano Dura-Ace 7900 10 speed cassette with a ratio of 11-27 cassette compatible with a dura-ace 7800 rear derailleur? 

Some people say its fine but more important to me is what Shimano says on the matter. Some online retailers specifically state that the DA7900 11-27 ratio cassette is not compatible with the 7800 rear derailleur.

I believe that I saw on Shimano documents that it was not, due to chain wrap problems occurring but cannot find that material through google so am reaching out to you for help.

If you could direct me to material relating to this compatibility it would be very much appreciated!
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