Clara is one of the most difficult roles to sing, not because her range is incredibly difficult, but all of her songs sit so much in the break of a soprano's voice.
You must be able to navigate through your chest, mix, and head voice, and make concious, deliberate decisions about where to place things. Kelli O'Hara sounds absolutely divine, but you know she was working hard to sound so effortless.
Those intervals in Light in the Piazza fit perfectly with Clara's character... she's reaching, rising, straddling childhood and adulthood... it's perfect writing.
I'd say that Guettel is one of the best melodic writers of our time. He has the sweeping romanticism of his Grandfather but with a distinct modern voice. He creates some of the best melodies that I have ever heard.
Specefically, Hero and Leander, Light in the Piazza, How Glory Goes.
Collins and Piazza sound NOTHING like each other. Maybe there's are similiarities in Dividing Day, because he's writing in Margaret's South Carolina voice, but I think it fits perfectly into Piazza's score, and could never be used in Collins...
Personally, I think I have too much bloom.