Most of the bikes I see with down sloping stems don't have the stem inserted all the way into the frame and this is true for mid to large frames where there is still plenty of room in the headtube for further insertion. To my way of thinking, it's better to use a Cinelli 1-A with a normal slope and insert it further down. Feng Shui tells us that an upward pointy object in your face will only cause unconscious fear to develop in the athlete and those downward sloping stems give me the willys.
If you need to get down further then why not bend your elbows?
Garth Libre in Miami Florida USA