Tire size

Yes and no. It will depend very much on what kind of surface you'll be riding. On a smooth, hard and even surface, the narrower tire will likely make you bit quicker or at least make you feel a bit quicker. On a rough surface, though, a narrower tire may actually slow you down because it doesn't roll over rough spots as well, so you'll be more inclined to slow down to deal with them, not to mention the narrow tire will make for a less comfy ride, again tempting you to slow down. On a soft surface, the narrower tire will also dig in deeper and actually increase pedaling effort needed, not to mention causing fishtailing and other stability issues. As a general rule, base your tire width on the surfaces you'll be riding. When in doubt or when exploring new trails, I always opt for wider. A wider tire can handle a much greater variety of surfaces.

From the perspective of mass and aerodynamics the simple answer is yes. As pointed out above...there's more to the equation than weight and drag statistics.