First of all, Lea recently recorded the first non-OBC recording of "For Good" with fellow Kim Jen Paz (who is currently on the national tour of Saigon). Lea sang the Elphaba part and sounded great. Granted, that song doesn't go as high as Defying Gravity or No Good Deed, but she sounds great.
I think we need to remember that many times people will change the keys for stars when they come in. Look at Aida when they brought in Toni Braxton. The producers of Wicked even changed the keys for Joey McIntyre when he came in as Fiyero. As long as they think someone is right for the role, they'll tailor it to them vocally.
Now, I'm not saying that she would be amazing. I don't know how good she would be, because I'm really not that familiar with her. I do disagree, though, with the idea that Elphaba can be sung as a legit musical theatre voice. All her songs were written to be Stephen Schwartz belted power songs. To do them in a legit voice would be the weirdest thing in the world, as that voice wouldn't match the melody or the orchestrations of any of her songs.
Lastly, if someone can sing high in their legit voice (i.e. Tuptim in King and I), that doesn't mean that they can belt high.
Eeeeeeyyyyyyyyaaaaaaaannnnnddddd aaaaaaaiiiiiiiiyyyyyyaaaaaammmmmmmm teeeeeeeelllllliiiiiinnngg yyyyooooooouuuuuuuwwwaaaahh...
