I have a Dura Ace std rear derailleur currently with 11-23 cassette, I'm planning on doing a very hilly race in the mountains in December and thinking I should get 12-27 cassette - is the std DA derailleur compatible? Running DA front derailleur with 'standard' DA cranks (dunno the number of teeth, never bothered to count them!).

Alternatively I was thinking of swapping over the 105 compact crankset (and maybe front Ultegra?) and swap 12-25 cassette from my other bike just for the race.

It would be easier just to change the cassette and not cannibalise my other bike but only if the derailleur is compatible.