Shortening the barrel reduces velocity and the trajectory becomes less flat but that in and of itself doesn't reduce accuracy I don't think.
Reduction in velocity results in less accuracy at great distance because a 9mm round from a 7" barrel is not going to have the velocity and not going to go quite as far as a 16" barrel.
The main thing is that the sight radius shortens unless you're using a scope.
I don't think the accuracy would be reduced to that of most pistols because most pistols do not have fixed barrels. A fixed barrel is going to be more accurate.
I am guessing also that the mass of a AR SBR chambered in 9mm is going to be more massive than the average pistol and this equates to less over all movement of the weapon from the moment the primer is struck.
Overall I think it's going to be more accurate than most 9mm pistols maybe with the exception of the very high-end competition and target model 9mm pistols.