Try a 105 11-28 cassette. They are relatively inexpensive. Weigh only slightly more than the Ultegra. In my experience 105 shifts and wears just as well as the more expensive Ultegra. You can even install it yourself with some readily available inexpensive tools. See the Park Tools web site for directions. If the 28 is not enough, many people have reported success with larger cassettes like the above mentioned Apex with short cage Ultegra RDs even though it exceeds the published specs. This seems to be a function of RD hanger length and a willingness to accept some chain slack in the small/small combination that you should not be using anyway. Also make sure your chain is long enough for the 28. I had no problem swapping the 11-25 that came stock on my wife's 566 with a 12-27 without changing the chain. Made a huge difference in her hill climbing. I personally don't like the large jumps between cogs when going to a wider range cassette and would make the change to a triple instead of anything over a 28.