all of those little changes were made just to get the car to as close to as possible of the 2017 rules, but because of obvious reasons, ferrari can't make a 2017 spec car and do testing with it, yet. so they've just modified it to sort of represent the sort of downforce that the cars will have next year.
the rw is just wider, but not lower (as they're going to be) because the aerodynamics of this car is basicly designed for a high rw. they would also have to make a new floor, to fit a 2017 spec wing.
there are skirts to generate more ground effect, because the diffusors are bigger next year. and i bet ferrari didn't want to, or wasn't allowed to make a new diffusor or a floor, to 2017 spec.
if they would've slapped on 2017 spec wings and floor, the rest of the aerodynamics would've needed redesigning.
what they did now, was they used tricks to get more downforce and ground effect, by just adding aero bits on the original car, to simulate 2017 specs.
this was a 'cheap' and functional way to test the tires, because they can collect relative data that can be comparable.
i have dyslexia and english is not my native language. please be gentle.