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It does not matter what caliber the FBI chooses, because statistically you are going to have to shoot a bad guy multiple times with a handgun anyway, regardless of chambering. The reason for the FBI going from the 10mm to the .40 to the 9mm is probably due to rank and file agents not being able to handle the recoil and size of the handgun and ammunition costs more than "stopping power". The FBI Agents I have met over the years have been mostly Firearms Instructors and and FBI SWAT/HRT guys who loved to shoot and had no problem handling 1911 .45s or S&W 1076 10MMs. Some of the rank and file agents who did not shoot a lot had issues qualifying with anything including the P226 9mm.

If you shoot a .40 or .45 well, you will shoot a 9mm better. IME the .40 has more muzzle blast and felt recoil than a .45, especially in a Glock. The .357 Sig is a good cartridge but IME is very hard on guns due to high operating pressures resulting in things like hairline cracks around the ejection port on high round count guns.

It is easier to shoot a 9mm faster and more accurately under stress than a .45. More leak points in a shorter amount of time equals faster incapacitation.

You can shoot a 9mm cheaper than a .40 or a .45. Cheaper ammo equals more shooting which equals better proficiency. As mentioned with current JHPs there is not a lot of real world difference between 9mm, .357 Sig, .40, or .45.

Many Tier 1 type instructors like Larry Vickers, Ken Hackathorn, and Chuck Taylor who favored the 1911 .45 now per their own published articles carry a 9mm Glock more than a 1911 .45.

These have been my observations in research and actual shooting.

Just my .02,
LeonCarr