You're right on both accounts. I was going to argue that it was being calculated anyway, but looking at the hitTest implementation I see that the area is only calculated if a ratio is passed.
Perhaps it could become a new function, getOverlapArea or some such? It goes without saying, I defer to you as this is one of the most well made libraries I've used (thank you, btw!). Not sure if percentage would be as necessary since area could also be used for comparison between the drop-zones to determine dropspot "most-over."
In the meantime, if anyone needs it you could get the overlap area of two elements with the following function (borrowed mostly from Draggables own hitTest implementation):
*Note: this is not as backward compatible as gsaps own hitTest code, so use at your own risk:
getOverlapArea = function(element1, element2)
{
if(element1.jquery) element1 = element1[0]; //convert from jquery to dom element
if(element2.jquery) element2 = element2[0]; //convert from jquery to dom element
var rect1 = element1.getBoundingClientRect();
var rect2 = element2.getBoundingClientRect();
var xOverlap = Math.max(0, Math.min(rect1.right , rect2.right) - Math.max(rect1.left, rect2.left));
var yOverlap = Math.max(0, Math.min(rect1.bottom, rect2.bottom) - Math.max(rect1.top, rect2.top));
return xOverlap * yOverlap;
};