FWIW, it runs fine with +P. I've put over 100 rounds of my carry load (Federal HST 124-grain +P) through my 99C with no issues. The first 50 rounds I put through it were MEN NATO-spec 124-grain hardball and it runs well with Winchester NATO-spec hardball too. It also works with relatively anemic stuff like Blazer Brass 115-grain hardball.
+P ammunition tends to be defensive ammunition, which tends to be expensive. Most people don't have the funds to wear a pistol out using enough of this ammunition to cause issues, and these pistols have proven to be very durable and reliable in the long run.
I'd suggest shooting as much as possible. By the time you do get to a round count where the +P designation actually makes a difference in regards to durability, I bet you could have purchased over a dozen extra P99 pistols with the money spent on +P ammunition.