The reasons most LE Agencies started carrying JHP rounds in the first place was less for knock-down power than it was to prevent shooting one or two people AFTER the FMJ round goes completely through the BG's body and emerges out the other side.
In my hometown, before they changed to JHP ammo for the PD, there was a BG who walked into the PD one night and started a firefight with the one cop who was in the station at the time. Officer fired 6 rounds, all of which hit the BG and exited his body, passed through a window out of the building, 3 of them coming to rest in the lobby of the bank that is directly across the parking lot from the PD, and the other 3 crossing the street, passing through the window of a church, and coming to rest in various pieces of furniture in the pastor's office. Needless to say, it didn't take long for them to switch to JHP ammo.