Smyth,
There would be no problem shooting at a 40 yard deer-sized target. Probably not a major concern, because most target shooters won't choose this site and most hunters won't be taking 40 yard shots.
My own groups at 40 yards haven't changed with this site, though I can now take them much nearer to darkness.<img src=icon_smile_wink.gif border=0 align=middle>