I got some 3/32 rubber cord and tried it out. It's 70 durometer. It is too flexible to use by itself. I got a 5/8" long 3/32 steel dowel pin and cut a short piece of the cord about .200 long. I put the cord to the bottom of the hole in the rebound slide and the 5/8 pin on top of it, inside the rebound spring. The exact length of the cord part will need to be adjusted for the feel to an individual shooter.
The end result is a "soft landing" for when the hammer is released, instead of a hard stop. With dry firing, this appears to help keep the sight on target as the hammer falls. It remains to be seen how this works in real life. It seems to be one of those "can't hurt, might help" type situations.
Edit- In shooting this setup in matches, I'm getting better hits at speed. Not dime sized groups, but enough better to tell a slight difference. Every little bit helps.