I cannot speak with total authority on this matter, but if your P99 cannot handle +P ammo on steady diet I'd be highly surprised. Most European amoo is pressure tested slightly differently than the American manufactures do and this results in their ammo typically running slightly hotter than U.S. ammo. In addition, I'm almost certain that a pistol like the P99 was designed to handle NATO spec ammo which can run at a maximum pressure of 42,000 psi as compared to SAAMI spec'd +P at 37,000 psi (or thereabouts). Either way European NATO fodder is often hotter than U.S. +P ammo. Most gun makes don't specify use of +P ammo in their manuals just like reloads because they don't want the liability of a wide variety of out of spec ammo causing warranty repairs. If in doubt call Walther. Shooting hotter ammo will wear the pistol faster however.
JMO
SS