Found this elsewhere:
This discussion comes up frequently in the Forum, and as such makes an appropriate topic here. +P and +P+ ammunition, though not recommended for use in Hi Power by Browning, has been used in small quantities without adverse affect, and in larger quantities to properly prepped guns without issue.
"Browning...... forwarded a copy of an internal test report in which it fired 5000 rounds of Remington +P+ 9mm ammunition in a Browning Hi Power pistol. "Inspection of the Hi Power system reveled no unnatural wear to the locking surfaces or any other area. Headspace was checked and found to be acceptable." The conclusion: "...the 9mm Hi Power system appears to be durable enough to withstand long-term shooting of the new ammunition from Remington."
Please note that extended or steady use of +P and +P+ ammunition will accelerate wear in modern Hi Powers, as it does in nearly every other handgun. The use of extra power recoil springs (such as a Wolf 18.5lb variant for 9mm) or recoil buffers such as those from Shok-buff can counter this pounding, and add service life to those pistols using the higher pressure ammo. Use of +P and +P+ ammo should be avoided altogether in older Hi Powers however, particularly early WWII specimens. I myself restrict usage of +P ammo to my newer MKIII variants only."