Plugs should still be good, K&N drop in is pretty much worthless or is a marginal gain, CAI is good if you tie it together with a quality exhaust and a tune. Plenum spacer will increase power, decrease MPG.
I wouldn't say decrease on the spacer. First couple of weeks yes. Because you going to smash the pedal to try it out. But after that phase it should be neutral or aid in mpg. Your engine will be running more effectively. The whole reason why we are getting higher mpg car is because they figured out how to use the engine more efficient. The tune after mods should help the most because these motors are so lean from the factory.