Specification here: https://www.jacksonsart.com/no-8-filbert-jacksons-black-hog-bristle-brush
The late Syd Edward was an admirer of these brushes. This filbert is quite long, and softer than Chunking bristle - it's said to work well with both oil and acrylic, and this sort of size should suit you well for the dimensions of support you have in mind; with perhaps a few smaller ones.
Is it stiff - well; stiffer than sable; stiffer than most synthetics; not as stiff as traditional hog. I wouldn't actually go to the wall in defence of any specific brush, because you have to get a feel for all of them, and whether these will suit you or not depends on a number of things. For acrylic, though, I would usually employ synthetic brushes, with a few hogs (any type) in reserve. And of the synthetic ranges I'd use, I'd go to the Rosemary & Co website, and choose from their Ivory, Shiraz (used to be Reflex), and golden acrylic ranges. But a handful of black hogs is always good to have - haven't used them for a while now, as it happens, but I doubt you'd be disappointed in them.