Feet per second isnt whats important.. the longer barrel makes the gun more accurate at longer ranges.. "Pretending to have a SMG" has nothing to do with it.. shooting a rifle for the cost of 9mm ammo is whats good about themThe longer barrel of a pistol caliber carbine seldom gives you any significant velocity increase. Here is an example where tripling the barrel length gained 15-fps:
More often than not pistol caliber carbines are purchased so the buyer can pretend he has a submachinegun.
Granted, a 9mm version of the AR can be useful for indoor practice at ranges that cannot or will not accommodate rifles. Otherwise I see little use for something the size of a carbine that shoots pistol ammunition.
EDIT: btw I highly doubt the rifle shot EXACTLY 1000fps and the handgun EXACTLY 985..