I have a 105 rear derailer with a 12-23 cluster on the back. I'm running the longest chain I can and have the b-screw maxed out. I was wondering what the largest cassette was that I could run. I have a race up some mountains in Boone, NC, and I know that the 39x23 combo isn't going to suit me well. I'm trying to decide between going with a 12-27 Ultegra cassette or if a 11-32 shimano 9 speed cassette would fit. I'm thinking about just canabalizing the two into one cassette so that the gear changes are smoother. I'm no slouch in the ITT, but being 6'2" and 185 pounds I'm having trouble keeping up with some of the other guys on the team when things get steeper than 10%. Some of the grades will hit 18% for short distances.