Okay, I just had a chance to play two more V-braced guitars: the Builder's Edition K14 and the 914. Plus I took another look at the K24. Some thoughts:
The 914 was just wonderful, with that rosewood growl we've become accustomed to, plus the type of improved balance since Andy revoiced the 900 series. I wasn't able to do an a/b test with a pre-V 914, so I compared the new one to an 814, which feels like a fair comparison. As far as intonation up the neck is concerned, the V-braced model wins the comparison hands down. I played single notes up the neck and capo'd, and in both cases the V-braced 914 was spot on, whereas the 814 seemed to get more out of tune (particularly the G and B strings) the higher I got up the neck. (Keep in mind, the degree to which a non-V-braced guitar has intonation issues may be unnoticeable to some people and downright offensive to others.) Personally, I'm constantly fiddling with tuning whenever I capo my 814 DLX, so the new V-braced guitars are something of a revelation (with one exception: see comments below on the K14). As far as sustain goes, I noticed no difference between the two guitars.
The Builder's Edition K14 (with torrefied sitka top -- note that my spell-checker insists on changing it to "terrified," which may be appropriate in this case) was an enormous disappointment. While absolutely beautiful, with both an armrest and a finger rest, I felt that the guitar was flawed. Not only was intonation up the neck not improved, it was terrible. I'd have been suspicious of the capo I was using if this had been the only guitar I'd used it on, but I also used it on the K24 and the 914. To make sure it wasn't something I was doing wrong, I took the capo off several times and readjusted the spring tension. No love. Every time I slapped the capo on, the guitar went out of tune. If I hadn't played the other two guitars, I might have come away believing that V-bracing was all show and no go. But the K24 and 914 were sublime. So my conclusion is that this particular K14 is a flawed guitar. Played with open chords, it was wonderful. Mix in a capo and stir, and the results aren't tasty.
If I were dying to buy one of these three guitars, I'd probably opt for the K24 because it offers a considerably different sound from my 814 DLX, while improving on the 814's intonation and aesthetics, the latter of which is highly subjective. The all-koa K24 is just a gorgeous-looking and -sounding guitar that offers a voice like nothing I've ever owned. One nit-pick, though, on the K24 is that it features Taylor tuning machines, rather than the Gotohs featured on the other two models. But then the K24 is also 500 bucks cheaper than the other two. Still, at this price, you don't expect Taylor to scrimp on the details.