i spoke to my mates friend who does performance stuff for cars and he goes the front splitter for powerfull cars it helps the aerodynamics but for a micra it drags the car and the aerodynamics don't work for a micra. that makes it drink more fuel
there is a difference between a well designed splitter and a lip spoiler.
a splitter will literaly split the air going over and under the car more efficiently that the bumper of the car allowing the air flowing under the car to speed up / mainatin its speed and reducing its pressure. this reduced pressure literaly sucks the car to the floor (ok so its the pressure above the car pushing down on it, same difference)
a chin or lip spoiler (like the laguna one) will do nothing for the performance of the car so far as aerodynamics are concerned but what it does do is reduce the air pressure directly behind it (usualy under the radiator) sucking air through the radiator and under the car.