I went from the Stock plugs to NGK Iridum plugs, my stockers were pretty shot. But I picked up about 20 miles on a full tank of gas after the replacement, probably due to the old plugs being way past their prime.
The 45 to 50% better gas mileage claim is deffinately not based on tests with the Nissan Engine in your truck. I'd stay very clear of any unknown brand of plug claiming outrageous increases in gas mileage.
Maybe you should go and get one of those Tornado gizmos too, combined with the plugs I figure you'll get about 90% savings if you go by the Manufacturers claims. 90% more efficient than 15mpg - Woaaa, that would make your X do like 135mpg or somethin!