Wow this thread got big fast. I think I'll reiterate the point I made on page 1 that weight loss has the most significant performance benefit during cornering and braking than it does in acceleration or speed. You can make anything go fast, including a 60 ton tank, a train, or an asteroid. Making it go around a corner fast is the challenge.
This is why race cars and motorbikes are obsessed with not only weight loss, but centers of gravity. You want the weight of the car to be as low, and as centered between the wheels, as possible. This is why almost all Ferrari and other super-high performance cars are mid engined. It's also why "boxer" engines exist. Obviously power/weight ratio is also important, but not nearly as much as cornering ability. Realistically, a Formula 1 car is actually pretty heavy because it produces a colossal amount of downforce at speed which adds a sort of artificial "weight" to the car, but that downforce isn't actual mass, so they can still take a corner faster than any car on earth because it isn't affected by G forces.
It's easy to add power to compensate for weight, which is why reducing weight for no reason other than to go fast in a straight line is retarded. But it's extremely difficult and requires a massive amount of engineering to figure out how to make a vehicle corner as fast as possible. This is why Ferrari spends like 280 million dollars PER YEAR engineering their F1 cars, and why drag racers don't. All a drag racer has to do is to just add more power.
tl;dr: if you aren't taking corners at high speed in your car it doesn't matter.