Just replaced my worn plugs 2 weeks ago with iridium NGKs and noticed a difference in idle and acceleration. Gas mileage seems to have improved as well.
Not making an argument for one over the other. Merely stating that the car responded well to the NGKs.
It seems logical that the car would run better with new over old plugs but unfortunately, that's not always the case. Especially if the new plugs aren't properly gapped. Fortunately, the NGKs don't have this issue.