Originally Posted by
genejockey
You have to get the correct generation of RX100, but yeah, A551 STIs should work with an A551 RD, and I THINK those are both compatible with 1055 (105) or 6400 (Ultegra) STIs and RD.
My understanding is that the actuation ratio of all of these rear derailleurs is the same, but the RD-A551 has a longer parallelogram than the RD-A550, in order to reach across an 8-speed cassette...