No an overdraw is to move the arrow rest contact point with the arrow closer to the nocking point (they were usually up to like 100mm further back - so you could loose 2-4" off an arrow)) - so for a certain draw length you'd have a shorter arrow (draw length in archery is usually constant for the shooter (apparently regardless if you use a release). They have become less popular as they can emphasise errors (setup is critical), but mainly because modern bows performance has improved hugely the last few years.
Generally lighter and shorter is faster for arrows.
Friction - Air resistance - as well as target contact resistance (better penetration).
Its an interesting idea - as the forces with a slingshot is linear (unlike bows) - it may work well. It worth looking at I think.
One point mentioned in an archery forum - is if you do use and overdraw that your hand is protected in case an arrow dismounts the rest on release...
If you google search archery overdraw you'll find info.