You will not harm your gun shooting conventional lead slugs through a modified choke, I can't speak to the new style sabot slugs as I haven't used them. With the standard foster, Brenneke, or Challenger slugs, you might even detect a slight improvement in accuracy with your modified choke over a cylinder bore or an improved cylinder choke. But remember, all gun barrels are individuals, and you will probably see a significant difference in accuracy from one brand of ammo to another. All you can do is go to the range, do some shooting, and record your observations.