Don't believe what you hear. Go pattern you shotgun with the factory chokes you might be surprised. Don't try just one brand of ammo. I've seen where switching brands will either increase or decrease your pattern percentage. In my 870 I use an improved modify and don't take it out. It gives me nice even patterns with the high speed shells I use. All my loads are over 1600 fps. The only shells that I use that are slower are my loads for geese and they are 1550 fps.
My best percentage is at 82% with steel 3's at 1710 fps with a 1 1/16 oz load. My goose loads pattern at 77 % with a 1 1/4 oz. load. These are reloads.