For what it's worth, I've done some work reviewing the changes in both 2.0.18 and 2.0.19 (though admittedly mostly the former). It doesn't look like we're missing out on much. I am particularly grumpy with how SMF handles security patches. They just say "security improvements" without elaborating further, and in the past we've seen them introduce more bugs than they fixed, even in vanilla.
I might have some time for testing early next week, but I wouldn't want to make that a promise.
As for 2.1, I think it might actually be easier to backport any individual improvements that you think may be beneficial. I've made a couple of (very small) contributions to that project, and I've watched it closely. The code quality is not good, even by SMF's standards. It might simply not be worth it.