So, I went ahead and put in an inquiry form about getting a custom McCall saddle made for Sierra.
As much as I love the saddle I have, I'm just not 100% satisfied with the fit on her. There's still some side-to-side roll and, while it's fine for arena work and local trails, I worry about doing 25+ miles with a not-so-perfect saddle fit. Plus, it makes getting on from the ground a bit wiggly and she gets upset at that. And I think I just need something that can compensate for her low withers a little better. Just all around, I want her to have, not just an 'okay' fit, but a GOOD fit.
I know the waitlist is probably 12+ months long, so I figure I'll at least get in the queue now. And by the time my number is up, she should have fairly well developed muscles from training and we should be able to get a good long-term fit for her based on that.
It'll be pricey, I'm sure. But it'll be worth it to have a saddle that is custom designed to fit both she and I perfectly. And a saddle that will probably outlive the both of us, too.
So... form is submitted. Now I shall wait to hear back from them about lead time and deposits and other details.