I'm fairly certain TGO and Loves2Shoot are correct.
I could be wrong, but I believe the same amount of energy (amount of gas) is released from the 9mm round, regardless of barrel length. The energy input into the system remains the same, but the gases are used more efficiently in a longer barrel. You merely improve the energy transfer into the projectile.
If I am wrong, please recommend a 9mm pocket pistol with less muzzle flip than my XDm 5.25.