Hi guys
I would like to share with you the happy end (it looks like an happy end, so far) of the story.
In these weeks, while exchanging messages with some of this board's members (thanks!!), I contacted Taylor USA and Europe to ask for assistance: I explained the problem which afflicted my 312CE. They were super nice and available to help me, but basically they replied that:
- being the warranty expired, it was not possible to claim that there is a manufacturing defect
- the guitar ships with Elixir HD strings and I had to install those strings in order to have a correct setup (uhmmmm..)
- an inspection could be done by technicians at Talor Europe, but at my cost.
So I put the guitar in m car's trunk and drove to a local luthier who had already fixed a guitar of mine. This guy is a qualified and seasoned luthier with a depp knowledge of acoustics, wood, physics, music. Plus he has a nice attitude and patiently listened to my story then started to carefully inspect my guitar. He told me that he wanted to keep the guitar for at least 2 weeks to find out how to fix the problem avoiding big works (live shaving the braces or modyfing the top).
After exactly 2 weeks he asked me to come to his lab. He played and checked the guitar for two weeks, inspecting every inch. He replaced again the saddle with a new buffalo bone saddle adapted to fit my Taylor, then fixed a loose cable inside inside the body and... surprise! He managed to significantly improve my Taylor's sustain and resonance. Now the soudboard resonates and vibrates as it should do, the sustain is longer and stronger, the tone is crisper. Even if he didn't even replace the strings, worn out and dirty, the sound improvement was remarkable. This is the short version of the story, but should someone be interested in the details, I will be happy to disclose them!
Last but not least: the final cost was so low that I instisted to pay more.
I was so happy and satisfied!

Some final considerations.
1. In this post I am not saying that Bob Colosi's saddle was not good or effective. Don't take me wrong. I respect Bob's work. Maybe that saddle was just not the ideal component for my Taylor. I installed that saddle on my Ibanez, and it works fine.
2. Elixir are good strings and Taylor co-developed a model with Elixir. But I wonder why Taylor official response to fix a problem was always: "install Elixir strings". Every single time. Any acoustic guitar should work correctly with any make, model and gauge of strings (within specifications, of course). If a guitar sounds decently with only one kind of strings, well, there's something wrong with that guitar or it is just a marketing statement

3. I am happy I found this great luthier in my town
Now it's time to stop talking about harware and start playing some good tunes with the band

Many thanks for your attention and suggestions!
w