I got to use my D-100C before it went off to Guild for some repair work for just a brief period of time, but hopefully I can answer some of your questions.
My D-100C, while having a similar feel of a D-55 (or a DV-72/73 for that matter), has a few manufacturing differences, from what I've heard.
The official Guild description lists the wood in the D-100 as being the "most select AAA spruce and finest rosewood" they had to offer and I can tell you that it sure looks that way, although interestingly the Guild literature also says the DV-72/73 had AAAA spruce.
As far as sound goes, my D-100C (even before the neck reset, etc...) had a deep tone, wonderful sustain, and high volume output. I can only hope that when it comes back it will be even better. As far as I know, with the exception of the carvel heel and supposed wood selection, I'd assume it has very similar construction to that of a D-55. I do note, however, that the nitro finish on mine seemed significantly thinner than on my D-55.
Now I'd like to make a comment about your experiences with your prior Westerly Guilds. The general consensus is that Westerly Guilds were built like tanks, which is a good thing... until over time the top deflects a bit and the saddle height becomes too low to actually move the top the way its supposed to move. What I'm getting at is this: A solid built Westerly needs adequate string break angle to drive the top in a satisfactory manner. The only disappointments I've had from Guilds in general (I've seen Tacomas with the same issue, of course) has been when the string break angle was too low to produce the sound the guitar was capable of making.
This is why I am ever vigilant on saddle height and break angle on any guitar purchase.
FYI: My JF-100C suffers from this problem at the moment. I'm saving up for the neck reset its going to need at some point. It sounds great as it is, but it lacks the deepness that I know its capable of.
Just my 2 cents.
