I have fired a Colt 635(9mm SMG) and a 14.5" 5.56mm with pencil barrel AR carbine(same N type butts and basic external layout, only major difference was caliber and internals to get there) I perceived more muzzle flip from the 9mm version than the 5.56(obviously nothing major from either, but more from the 9mm never the less), although the 5.56 was much louder.

9mm is also supposed to be somewhat harder on certain AR parts than 5.56 guns. Colt 9mm hammer pins are stainless instead of using their standard pin from the 5.56 guns as an example. I would assume any of those traits could be enhanced by way of the .40(although Olympic Arms uppers are typically described as "plug & play") but may be some points want to research while making your decision.

"My message to my troops is if you see anybody carrying a gun on the streets of Milwaukee, we'll put them on the ground, take the gun away and then decide whether you have a right to carry it." - Milwaukee Police Chief Ed Flynn