Sounds like the night sights weren't set correctly for the gun, with the front being too tall (or rear too short). Usually, aftermarket sights are made so they can be fit/tailored to the gun, with the front too tall. The gunsmith is supposed to file it down so the gun shoots point of aim at X distance. Night sights are a little trickier, depending on the type, as they often have an outline shape and glow portion to align with the rear, so you have to be more attentive to adjustments.
Were the night sights put on by factory, or a wanna be gunsmith?
I once shot with a fellow who complained of his pistol shooting low. It was his first .45. He made the jump from .22 to .45 as his next gun. I tried his pistol and put everything grouped about the X ring.
I let him shoot one of my 357 revolvers and only put 3 rounds in the cylinder for him at random. I watched as he squeezed off rounds, and the problems was him. He was doing an anticipatory flinch upon trigger release, whether a round went off or not. You could see the barrel drop when the hammer dropped on an empty round. He was very consistent, as his three shot group could be covered with a silver dollar. But, they were all about 3-4 inches low.
I haven't shot my SIGs for a few years. I actually have two. They come with a 10 Meter test fire target from the factory. One of mine shot about 1/2 low @ 10 meters and the other was about 3/4 inch low at the same range. Grouping on one was about 1 inch and the other 2 inches. I remember shooting one ragged hole indoors at 25 yards a little over 2 inches in diameter, with eight shots. I was impressed with that being a totally stock gun. I couldn't do that with a 1911 that wasn't accurized first. Back then, I could maintain focus on target, front, and rear sights simultaneously. Now I can only focus on the front sight. Aging is cruel.
For semi-automatics, the springs for the slide and firing pin must usually be changed for +P loadings. I also recommend a shock buffer to keep from battering the frame. Such modified guns then may have cycling issues with standard ammo.
The owners manual for the SIG 220 state the use of ammunition meeting SAAMI specifications. I think +P ammo exceeds that. But, I am not certain.
I don't know why you need +P ammo, though. You shooting through car doors or several interior walls?
Maybe what you really want is .454? That puts a neat hole in 1/4" plate steel. Not exactly a concealed carry gun, though.