well, what chainrings up front do you have?
consider a triple?
can easily get a 26T 74BCD chainring.
the jumps in gear ratio really suck for 11-34 and 11-32 cassettes.
+1
Mountain style cassettes are fine in the mountains but the large ratio jumps cause large cadence jumps and that sucks on high speed road rides, especially if you're trying to keep up with other riders.
Shimano mountain cassettes are 9-speed. If you have 10-speed shifters you'd need an IRD or some other aftermarket mountain cassette. How many cogs do you have in the back and what size chainrings?
It may be possible to convert your 12-25 cassette to a 13-28 if you can find the right parts. And a new 12-27 would be less expensive.
Al