In reply to tony:
From an older thread talking about whether to go for a compact or a 11-28 cassette:
To give you an idea of how worth it the options are:
Lowest Ratio at the moment is 39/25 = 1.56
With a 12-27 it would be 39/27 = 1.44 = 7.6% less force required
With a 11-28 it would be 39/28 = 1.39 = 10.8% Less force
With a compact (34T) and your existing 12-25 block it would be 34/25 = 1.36 = 12.8% less force required
11-28s ride ok, there's no massive gaps until you get to the softie end, and then it's quite welcome