Shimano always bases their maximum based on the group the component (RD's etc) has as it's maximum. At least according to Sheldon (Our Lord) For instance, Shimano's Ultegra GS and SS RD is specified as 27T maximum in the cassette/freewheel. And that's just bull. They will handle a 30T cog just fine - possibly more. Here is the Harris list with the largest cogs as stated by Shimano:

http://sheldonbrown.com/harris/derai...r.html#shimano

Happy Trails!