A few things - first of all, Ultegra and D/A can handle up to a 30 tooth cog. I have 13 / 30 with my Ultegra 9 speed and it works like an absolute champ. I'd recommend this gear if you want to do some serious climbing over the 27. The 27 is only 7.4% lower than a 25, so it's not a huge difference.

Second thing is that you will be surprised once you get the big gear that it's very easy to maintain your speed. When I ride with my buddies who have 25 rear gears, they can be standing and cranking away, and it looks like they should be pulling me, but I just sit there and spin my 85 rpm and hang with them easily.

Lastly, I was so happy with my 30 gear, that I went a step further and got a compact 50 / 34 crankset, which made life even better. Keep in mind that most of my rides involve a lot of climbing, but with this setup, I can ride over any hill at a comfortable pace and not burn out my legs. People say it's a granny gear - I say everyone else is over geared. I think I'm right because they seem to have a lot tougher time on the steeps.