Originally Posted by
rpenmanparker
... lowest gear to 34/28, you can get a total of 23% reduction. Assuming you want to stay with a road rear dearailleur and don't want or have Apex, that is the best you can do.
Shimano are moving to
30T cassettes. 105/Ultegra reportedly available later this year.
I'm running the 30T 4600 cassette + 5700A derailleur on my compact, climbs pretty nicely.
OP: a 28 cassette would be a good place to start.