I thought 27 was the max? Anyway, I tried a 30 with an older 6400 Ultegra tri-color RD. It shifted and ran but was not happy. I would not recommend it, but you could try it and see. Depends on the length of your b-tension screw and the geometry of your RD hanger.

FWIW I've always been able to get a 28 to work ok with a "road" RD.