Iridium plugs recommended to me by other MX5 Mk1 owners. A worthwhile upgrade. With these fitted pick-up is most definitely smoother and a reduction in the "stumble" on sudden throttle opening that these cars tend to suffer from sometimes. I like Denso plugs having used hundreds of them in many years I spent working in after sales in a Mazda dealership, never experienced any problems with them.
These definitely improve engine efficiency and fuel economy. They also last longer than cheap plugs so don't need replacing as often which is an added benefit.