Originally Posted by Davet
The standard Dura Ace front shifters are double only and the rear dérailleur may not work satisfactorily with the 12/27 cassette, there usually isn't enough adjustment with the B screw to keep the top jockey wheel from hitting the largest cog, and it may not have enough wrap capacity. Usually a long cage rear dérailleur is necessary. The rear shifter won't have to be changed if a long cage or MTB rear dérailleur is used.
Sorry... was having a brain fart. Realized after I posted that Dura Ace does have different front shifters for double and triple.
The DA short cage derailleur specs are the same as for 105 and Ultegra, which both work fine with a 12-27 cassette. I've never tried it with a DA short cage, though.