My team captain runs a 11-21 on his Shimano [rain] and Campy [dry] bikes. He too had the same problem about a cassette but bought a Ultegra 11-23 instead of the Dura-Ace 11-21.

BTW, He runs a standard 53/39 crankset and with all the hills around here, hates to use the 23 because he feels like he spins too fast. I have seen him kick most everyones butt on long grades of 6-9% mashing all the way. I need at least a 23-25.