A pistol firing 5.56 NATO? That's an interesting combination of small diameter, long chamber, light weight, and larger (?) powder charge.
A pistol brace would definitely make shooting a pistol steadier and probably more accurate. OTOH, it makes the pistol a bit less handy, and would complicate concealment.
IMO as a non-shooter, the whole pistol brace controversy is bureaucrats hating something that doesn't fit their neat little innovation-crushing ivory-tower categories. I doubt many mall, drive-by, or building-balcony mass shooters would use a pistol brace.