It seems to me that S&W has reduced the radius of the lead-in (not necessarily the depth) too much. In the photo I posted of the 627 it doesn't appear the stop rides down in the cut at all, but only along its forward edge. I'm thinking about looking up my local "gun plumber" and seeing if he can recut the lead-in (with a mill, I guess) to match the old style. I believe that would make a big difference.
I had this problem last year with a 625. In that case, among other things, the stop was rather rounded on its forward edge. I got a replacement which happened to drop in nicely and worked for a few hundred rounds. Then it turned out the new stop was so sharp that, as it bounced along the cylinder, it was gouging out bits of metal, leaving a line of pits.