I've said it was a 20%-30% chance to score. I've never once said it was a "slam dunk".
Yet it absolutely has to be a bigger % opportunity vs a player fouled as the last man who isn't in the act of shooting. You can't score a goal if you aren't attempting to shoot. (It's likely that the player is free on goal if he isn't fouled, that's the point of the last-man red card but there's an assumption that this is the case, there's an assumption that he would touch the ball around the last man and be in a position to score from, it isn't assumed that his touch took him wide, or that he would fluff the shot, or slip).