Rear Casette

Are you sure the cassette is toast? Try a new chain and see if you think you still need the cassette.Going form the 12-30 to 11-25 will be noticeable on hills, especially if you use the 30 a lot. The Ultegra will run just fine.

I have not noticed any difference between a 105 and Ultegra cassette. There is a HUGE difference between the 12-30 and 11-25 cassettes mentioned. Without knowing the chainring gearing makes giving advice more difficult. From my experience, a 50x34 front and 12-26 (SRAM 9sp) combo gets me over everything, and these are hills that most tri-events would not consider. If your average speed is say 20 mph, then most of the time you will be in 50 chainring and 17-21 cogs. A 12-30 cassette has too much gap in gear ratio in the sweat spot. At >32 mph, I run out of gears. So little time is spent at the extremes of the cassette range that the 11t cog is not needed by most people. Go with a 12-25 or 12-28 cassette.

I have a 50 32t in the front, i ended up ordering an ultegra 11-28 since there are probably only a few times a year i ever use the 30t. Im hoping the 11 will help me get a little more speed on some of the steep decents around here.