+1 on the Ultegra or 105 cassette. You should be able to find a 12x27 in either model if you want to stay with that range.

There is a very small weight penalty but the cost of either cassette is much lower than Dura Ace, there is no performance or shifting penalty and the larger cogs are more durable since they are steel instead of Ti. Unless weight (or snob appeal) is your overiding concern, either Ultegra or 105 is a more sensible choice.