Plugs and leads will not make any difference - no matter what youre told or sold.
The only time they can make a difference is when you require a hotter or colder running plug because of extensive engine modifications. You will lose power for fitting wrong ones, and gain it for fitting the right once, but it is not really a gain when youre claiming back lost power is it?
If you need new plugs/leads then fit what came with the car. A back box though, yea thatll make a difference