I think if .3" stroke is added to the slide by extending the frame rail guide, then .3" needs to be removed from the back side of the recoil plug tunnel. If this is done, then the only way to hide the .3" removed from the tunnel is to use a long dust cover frame and cut it back from the front just to hide the gap.
If you extend the rail guide and don't cut the recoil plug tunnel you are not getting the "stroke".