The P99 has been engineered with a very long dwell time. (The time and distance the barrel and the slide stay locked together while the weapon is in recoil). This allows the firing pressure plenty of time to drop to a safe level before the breech opens. The pistol also has a strong recoil spring. These factors combined permit the pistol to handle very high pressures and fire accurately. The pistol's glass-fiber reinforced frame and steel inserts allow the pistol to stand up to substantial recoil forces very well, too.
I believe the P99 can handle high pressure ammo about as well, if not better, than any other pistol design on the market. SIG's SP2009 is the only other pistol that I can think of that is as good or better.
I've shot a lot of +P and +P+ thru my P99s with no ill effect. It handles it quite well.