Dont know about the slide thing.
I had a couple of early 19's (Im calling them Gen 1's, as I got them when they first came out), and I currently have a couple of Gen 3 19's, and I like the Gen 3's a lot better than the early gun. The grip was (and still is) the main issue. Both generations work/worked fine, function wise.
The older guns had those "mag rip" cutouts (like the 17's have) in the front strap, which fell right across my pinkie when it was on the grip. The grip is cramped enough fo me, and I really found it to be annoying and distracting. The Gen 3's and later are better now that its gone, but they are still cramped to me, and Im going to file that lower edge flat when I get around to it and restipple. That lowest, "half" finger groove, is still in the wrong place for my pinkie.