I haven't quite finalised the title yet as I don't like the word 'devices' but the current draft is
"What is the most effective device in making a race car faster?"
Not very happy with the phrasing or the wording. Actually, if anyone can come up with a better sounding title then feel free.
If thats your title, then your conclusion will surely read:
"The most effective device in making a race car faster is every single piece of it."
as thats the answer

. Over the decades, almost every part of a racing car has been refined and transformed to make them faster. We could debate all day whether the tyres, engines, aerodynamics, brakes...any number of things were/are "most effective" at making cars faster.
The problem is this:
Wouldn't what contributes the most to race cars be different based on series specifications? For Formula 1 and even LMP1 cars aero contributes greatly. So for F1 you would be looking more at aerodynamic devices. Do you plan to break it down by series or just in general which devices are the best? Maybe I'm just confused.
As he says, the problem is that all motorsports have regulations that for the most part have attempted to:
1. Slow cars down for safety.
2. Make motorsport cheaper to compete.
3. Improve the entertainment.
This means that the most effective way of making a car faster becomes narrowed down to what can and can't be developed. In F1, the only thing left that the teams can really develop is aerodynamics and even this is heavily restricted.
But for Le Mans, the changing regulations means there is scope for engine development to gain time especially for a GT class car.
Its also very difficult to analyse particular parts being more effective when 90% of the time, the most successful racing cars have also been run by the most well-funded teams. Lets take engines as an example. The teams that have the most money have the most development spent on their engines. Generally this means that their engines are going to be the most successful almost regardless what other teams come up with simply because they can spend their way out of disadvantages. In the early 90s we had all sorts of bizarre engines such as the Life W12 but without the money to develop these engines, they could never make the engines work due to disadvantages in weight or size. A W12 was probably never going to be a championship winner due its weight and size but equally it probably would have got some decent results if it had had the backing and development Renault or Honda had at the time.
Its easy to look at the Ferrari F2002 and say "the aerodynamics of the F2002 are the most effective way of finding speed" but the F2002 may not actually be the best design, it was simply the most refined and tested design due to Ferrari's huge budget and endless testing. Someone feasibly could have designed something even faster but without the money to develop and test, its very difficult to say one way or the other.
The further back in history we go back, the slightly easier it gets because pretty much any innovation propelled a team from backmarker to championship winner. Then we can say "ok, so the carbon monocoque of the MP4/1 was the most effective way of building a chassis".
An interesting but very challenging topic you've chosen, good luck! There's plenty of stuff to debate with though, so it should be easy to write thousands of words of discussion!