I also had a similar setup to you (compact double with 12-25). I bought a 12-32 IRD 10 speed rear cassette and a medium cage 105 rear derailleur for about $160 + $25 install. With the 32 teeth, I did need to change from a small cage derailleur. I don't know if you would get enough lower gears out of one with 28 teeth to make it worth it. Now my bike has similar gearing to a triple (they have one lower gear). It greatly helped because it is very hilly where I live. I don't find the jumps between gears to be too big.