My experience with the Xenon is similar to Ryan's. I'm 5'9", 170, and typically wear a 15.5, 33 in dress shirts. The medium Xenon fits me well in the sleeves, neck, and torso, but I too feel that it is slightly too short. There is only just enough room to layer a baselayer and lightweight fleece underneath. Anything more would be basically impossible.
IMO the Boreas (also in medium) has a much snugger, longer torso, while the Infinity has a bit too much extra girth around the torso and is also notably shorter. All of them fit me, more or less, but I'd say the Xenon has a semi-fitted or slim fit, compared to the Infinity's more boxy cut.
Hope this helps!