Did you get a manual with your 915? If so, did you read what it says about ammo?
Specifically -
*Use only commercially manufactured ammunition with internal ballistic pressures which are in strict accordance with the specifications of the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI).
*Be certain all ammunition you use complies with the SAAMI specifications. If you are uncertain, contact your ammunition supplier for verification.
+P designation is a SAAMI spec so any ammo indicated by +P meets SAAMI.
Your 915 will tolerate more +P ammo than you are likely to shoot thru it in your life time. My dept use to issue 5904 (alloy frame S&W 15 rd) and the only ammo we used was +P+ which is hotter than +P and exceeds SAAMI specs. One of our range guys tried to wear out his 5904 by seeing how many rds of +P+ he could fire thru it. When he retired he had documented 65,000 rds of +P+. He only changed the barrel and springs once. He bought the gun when he retired and still shoots it. Knowing how much he shoots I would guess he has well over 100,000 rds thru that same guy by now. Remember - this is with +P+ ammo which exceeds +P pressures.
At this rate if you can afford to spend $20,000+ in ammo and shoot 100,000 rds thru the same gun then you've gotten your money's worth out of the 915 Value Line S&W.