No, I don't know about laser iridium plugs, but what I do know is that a car produces the best mileage with 100% factory settings and spares.
Adding anything 'after market' to the car hurts the gas mileage. This includes custom rims, different tire sizes, mufflers, special air intake/air filters, useless roof scope and spoilers, non-standard plugs etc..
The best mileage is (in my limited knowledge) produced by the car which is closest to factory defaults.