If your shirt is uncomfortable when you are sitting or moving around, it doesn't fit perfectly. Your shirt should be comfortable throughout the normal range of movement that you would expect to encounter when you are wearing a dress shirt. You don't need a performance fabric, you need a shirt that fits correctly.I'm going to agree with @medhat.
To point #2, I have a couple of bespoke dress shirts (not MTM).
While they are incredible works of art, and do fit well, they generally look and feel their best when I'm standing upright. If I sit down, or move around, they can feel tight around my mid-section or even chest, and as such, I tend to feel a bit more uncomfortable than I'd like.
For that reason alone, I don't wear them very much. Also, if I go up in weight some (even slightly), which is an issue I've dealt with since a child, the shirts fit too tight in the midsection.
It's a dress shirt, not a superhero costume.