I own two P320s, a compact 9mm and a full-size .45 ACP, and I shot a full-size P320 in .40 S&W on three separate occasions (50 rds of 180 grain FMJ each time).
I have had absolutely no reliability issues with the two pistols I own and had no issues with the 40 caliber P320 rental I shot.
My impression of the full-size P320 .40 could not be more different than yours. Considering that the .40 S&W cartridge tends to have sharper recoil than 9mm Luger or even .45 ACP, I thought the P320 .40 was very pleasant to shoot and quite accurate as well. The forty caliber was actually the first P320 I shot and my good experiences with that pistol prompted me to buy the two I own.
I have had absolutely no reliability issues with the two pistols I own and had no issues with the 40 caliber P320 rental I shot.
My impression of the full-size P320 .40 could not be more different than yours. Considering that the .40 S&W cartridge tends to have sharper recoil than 9mm Luger or even .45 ACP, I thought the P320 .40 was very pleasant to shoot and quite accurate as well. The forty caliber was actually the first P320 I shot and my good experiences with that pistol prompted me to buy the two I own.