I usually go with a large amount of water because you don't really need that much starch to act as an emulsifier, and generally for pasta, if you use relatively small amounts of water, the pasta will drop the temperature of the water and simmer awkwardly for a bit.

though I came across an interesting idea for pasta "Risotto style"... last night. using vege or chicken stock. apparently it'll thicken stocks into good sauces (not sure it'll work for mac and cheese, though. they say already thickened sauces like mornays are not so helpful.)