My situation is similar: Ultegra ten speed compact and looking for lower gears. I'm planning to get a Triplizer from IRD to turn my 48-34 into a 48-36-26(24). I will need to get a new RD -- probably a 9 speed Shimano MTB RD, but your long cage should be fine. You will need a new FD and a way to shift it. I'm using bar ends. For a cassette, I'll get an 11-36. Maybe overkill but ...