The effects of having an enlarged sound hole are controversial. In theory, it raises the Helmholtz resonance of the box (by a semitone or two), and thus enhances the mids and trebles. That would help explain the popularity of this mod for bluegrass, as enhanced mids and trebles equates to more cut when playing in an ensemble. But that's not what some people report hearing. Some claim to hear more bass, exactly the opposite of what theory predicts. Plus, a lot of fans of the large sound hole (luthier Bryan Kinsey, for example) like them because they feel it changes projection -- more spread out, rather than sharply focused in front -- not the tone. In particular, the player can hear the guitar better. (Which might explain why some dread players report that the change made their guitar bassier.)
My only guitar with a large sound hole definitely has stronger mids than its nearest small sound-holed cousins -- which is good, because that's why the guy who choose the specs went with a large sound hole -- but this isn't a model for which there is an exact small sound hole counterpart.
By the way, part of the reason the fretboard sticks out so far on the CW/TR D-28 is that the original fretboard was swapped for a longer Guild Gretsch [correction courtesy JT] fretboard back when Clarence owned it. (That poor ol' guitar has been through a lot!) Tony thinks that the extended fretboard partially mitigates that effects of the larger sound hole, and also that the larger sound hole doesn't make much difference in the first place. Like I said, it's controversial.
-- Bob R