No, I don't have one, so you can stop reading if you like
![Undecided :-\](http://www.unofficialtaylorguitarforum.com/Smileys/default/undecided.gif)
But I do have a 322 and simply wanted to offer that the Blackwood back and sides offers Taylor a tone dramatically different than mahogany back and sides. I've had 2 or 3 Taylor 12 frets that had spruce top and mahogany b&s and honestly I really couldn't tell they were 12 frets because Taylor brightened them up far too much IMHO.
The 322 is far from bright due to it's mahogany top, but the Blackwood b&s tempers that a bit so the overall tone has a great balance for me.
So when I read the 312 specs of Blackwood b&s with a spruce top, I started to drool a bit cause I'd love to play one. I imagine it being crisp due to the spruce, but dark and growly due to the 12 fret Blackwood back and sides.
Your dilemma is that just trying out a regular spruce & mahogany Taylor GC 12 fret won't do you the least bit of good, as I predict the Blackwood will give it a uniquely different tone from those.
Since I'm spending your money, I say go for it!