+P+ (just skip to the end for Growler's and Steve's answer)

Don't know specifics about using it in Sigs (warranty, etc.).

In general it is harder on the gun due to the increased pressure. +P+ will definitely feel different when shot (more recoil), may decrease your accuracy and time between shots.

In general, overpressure rounds (+P and +P+) will function fine in your SiG. I would not personally recommend feeding it only these types of rounds. I use 147gr JHP's rated at +P for my SD/HD rounds but use 115gr ball ammo (usually WWB or similar) for punching paper. I do ALWAYS end each range session with at least a few mags of my 147's to refresh my cranial as well as muscle memory of the differences in POA/POI between "range ammo" and the "good stuff". It also cycles through my stock of ammo so none of it gets too old

I think I answered on another thread about wear and tear and recoil spring replacement. Generally SiG recommends replacing every 5-10K rounds. The variables factor in with the ammo shot. Steady diet of overpressure loads would be around the 5K mark, sole diet of WWB would be around the 10K mark. Mix and match and you'll fall somewhere in between. Individually one would have to figure out where that point is to recommend a spring replacement.

The use of +P or +P+ ammunition does accelerate wear and reduces the service life on the component parts on any pistol.