You can expect exactly zero gain from spark plugs. All they have to do is ignite the mixture - they either work or they don't! As long as your ignition system is in good working order, leave it alone.
As for injectors, once again there is zero gain to be had unless you've extensively modified your engine to the point where the factory injectors don't flow enough.
I'll 2nd both those, changing the temperature of the spark plug might make a difference if they keep getting fouled up, but thats it