Tactically I promise there isnt any advantage to under loading. If I knew I was going to get into a SD situation when I went out the next time Id stay home.
But if I had to go knowing that I would be armed with 12 ga and 00 buck or an AR 15 or something and as much ammo as I could carry. And bring six friends similarly armed for bear.
The pesky part of SD is that you dont know if or when it will happen. Or what it will take to handle the situation. It could run the gamut of just having to draw a weapon and not fire to firing everything you have and still wishing for more.
Best advise I have to offer from my own experience.