I can only partly answer, but here goes. In order for the coffee grounds to truly produce usable food for the plant, they would have to be cycled through the micro-life that lives in the soil. In their raw state (i.e., just sprinkling on top), they wouldn't do much to increase anything. As for whether they will result in more nitrogen, I don't know exactly what they'd break down into, but, again, it'd be the microbes in the soil doing the breaking-down, which takes time. I'd suspect, though, that you might have some issues with pH if you sprinkle grounds on top and haven't taken steps to control the pH.
Cheers!