Not really, what you might be thinking of is the same panels that racing cars have fitted. These panels are there to speed up the air flow under the car, from the splitter at the front to the defuser at the back. It makes the car like a reverse airplane wing and offers down force.
For this to work though the car needs to have an almost uninterrupted flat surface from front to rear and needs to be travelling at fair old speeds. Somewhere in excess of highest speed limit for it to be of any use.
I disagree, road cars have to travel through the air too, and even though they are not designed to produce downforce, modern cars are aerodynamically designed to slip through the air to get a good fuel consumption, the undertray looks like it fills the space between the bottom of the front of the car and the front of the floorpan where it seems to line up nicely, although there are parts sticking out like the suspension, the exhaust, fuel tank and other components are tucked away out of the airflow
The undertray looks like it has aerodynamically designed vents in it and if you thing about it the air going under the car is going as fast as the air on top (eg, if the air on top is doing 70mph, the air underneath in doing 70mph)
Areodynamic drag is a square law - that means doubling the speed increases the drag by four times,(eg - there is FOUR times the aerodynamic drag at 70mph than at 35mph ) that is why the fastest Veyron had to have the power increased from 1000Hp to 1200Hp just to get a few more mph to retake the "fastest car" catigory - James May drove it but was not allowed to explain the square law - the producers wouldent let him, they thought the audience would not understand it - so he had to say the air was "thick" like treacle at high speed, instead
you can consider the undertray like the fairing on a motorbike, it too, does not produce downforce or does not go all the way to the back of the bike, but helps the bike slip through the air more efficiently - if you took the bonnet off, do you think that would make a difference to the aerodynamics? its like a bonnet, but underneath instead of on top!
It is hard to say how much of a difference the undertray/ fairing makes, remember these cars are also designed to run at 100mph+ speeds on Autobahns, where it could be very signifcant, but even at 70, it could make a difference
I remember reading that over 40mph, aerodynamic drag is important, at what speed do the active grill vents close?
I might put mine back on once ive fitted my new bottom engine mount and q-rusted and hammerited under the car - it only weighs 2.4kilos!