I actually used an Ultegra long-cage derailleur with an 11-34 cassette on one of my other bikes with no apparent problems. I didn't realize when I installed the cassette that Shimano rates the RD as 27 teeth max for the cassette. However, I don't have another long-cage Ultegra RD, so I would prefer to buy a derailleur that is properly sized for the larger cassette.