Before I joined SF, I wasn't anywhere near as picky about the way things fit me. I had RTW stuff tailored, but that was more about appearance than comfort. And there was a definite difference in comfort between when I was wearing a suit and tie and when I wasn't. Now, on the rare occasions I buy rtw, I am very particularly about ease of motion within it. As stitchy says, it depends on how well it fits. With most of my stuff, I can't feel a difference in comfort, even with my tie tightened and my jacket buttoned. That being said, it's pretty much all made for me (and all shirts are). I doubt I'd be saying this if that weren't the case.
And I'm willing to bet that if I wore braces, MC would be more comfortable than something with jeans