A couple of years ago Brownell's was selling an 870 after market trigger group with a fully adjustable trigger.
This was mostly for the people shooting slugs at very long ranges, where a good rifle-quality trigger would be useful.
I can't remember who made it, but it was a steel unit, and cost BIG bucks.
If you're interested, contact Brownell's to see if they still have it or can put you in contact with the maker.