www.sheldonbrown.com/gears
He also custom-makes cassettes and upto and inlcuding 30t sprockets WILL work with a short cage rear road mech.
Problem is , the bugger wants 100 bucks for the 13-30 cassette which is the sort of gearing i want on my double (have the same problem as you , and my bike was only available with a crappy 13-25 block)
I would try to get the cassette and/or rear mech fixed before touching compacts.You may need to adjust things like BB aswell if you change chainrings.
I'm debating 12-27 cassette but dont think it'll be enough for the climbs i do , so if i dont stump up 100 bucks for a custom cassette the other option is an 11-32 MTB cassette and a new MTB long cage rear mech (the last 2 sprockets on a 11-32 are 28 and 32.Would prefer a 30 to be honest - thats probably 100 bucks too...)
Seems like theres a massive hole in the shimano cassette market for 'alpine' gearing.
Campag have a 13-29 cassette for roadies that i wish i could use.
Still debating what to do , let me know how you get on.