Twist width is probably one of the most frequent "rider-fit" issues I run into. As with so many facets of saddle fitting, it's a very subjective thing; Person A's perfect twist may make Person B feel as though they're sitting on the narrow side of a 2"x4", while Person B's perfect twist may make Person A feel as though his/her hips are being torqued out of joint. I fall firmly into the "narrower twist" camp. I love my Passier GG, I love the Black Country Eden, and I'll even cop to loving the ride of the old, hard German Stubben dressage saddles (the Tristan in particular). However, my mare will turn 15 this spring, and has developed a bit of middle-age (read: hoop tree) spread ... No matter how much work I put into her, she'll never be the same shape as she was when she was 7 ... And given that I'm 50 and will never again return to my pre-childbearing 26-year-old size 8 shape, I don't feel as though I ought to be pointing any fingers.
Anyway, this means Lyric will be moving into a hoop tree, which means that I will be riding a wider twist. You see, twist width is determined by tree width AND by tree type. The rails on a spring tree should be at the same angle as the tree point; that means the wider the tree, the flatter the rails and therefore the wider the twist. In the photo below, the tree on the left is an extra wide hoop tree, and the tree on the right is a medium-wide standard tree. (Ideally, for comparison's sake, the two types of trees would have been the same width, but I'm working with what's lying around my bench!)
Here's a shot comparing the rail and tree point angles (hoop tree on top, standard on bottom):
Here's the twist on the standard tree:
And here's the twist on the hoop tree:
It's not a huge difference - roughly an inch - and for some people it wouldn't be an issue. But for some, their personal conformation would make it very hard to accommodate that extra inch. Hopefully I'll be in the former camp, but we'll find out come spring ...