If there were enough demand I am sure they could do it. You could always contact them and see if they would do a one off for you when they have time/next time they place an order. Alex is always willing to satisfy the customer, I am confident that he would work one up for you when they have time. I just don't know how much demand there is for mass production.
Normally I would want a 1:7, but since I already have a stack of ARs in that twist, I really don't care that the XCR is 1:9. In fact, I think 1:9 actually shoots 55-62gr more accurately (at least in my experience), so I'm glad 1:9 will be the first offering in my particular case. If I want to shoot heavier bullets, I have a gun to do that, as I'm sure most of us do.
A 1:7 SBR barrel would be a nice offering though, for sure.
I've got a 1 in 8 16incher myself. I don't get too bent out of shape about twist.
All guns are individuals. They will react differently to different bullet weights. Some 1 in 9 guns might stabilize a 75gr bullet; another might not. Your load will also be a big factore. More velocity will stabilize a long bullet in a slower twist--just don't blow the gun up.
Really, all you can do is shoot this stuff and see what happens. Then adjust accordingly. Yeah, you might neet a faster barrel if your present one won't stabilize your bullet weight of choice. You won't know for sure until you try, though.
Out around 100 yards I don;t find that much difference unless you're spinning a light bullet too fast and make it fragment. Out at 200-400 yards, twist makes more of a difference as the bullet has stopped cavitating and will be stable. Then, twist and length matter more.