Cars, especially sports cars, often have their horsepower compared as a somewhat ignorant method to determining the faster car. However, this is not the most accurate way of making judgement because there are other variables, some much more important, in determining a cars acceleration or top speed. While an engine's horsepower provides the force, accelerating the car, it is important to remember the basic fact that F=ma. Therefore, a 4000 lb car with twice as much horsepower as a 2000 lb car will accelerate at the same speed. This is the reason that lightweight sports cars that are often half the price of supercars have similar accelerations. Especially with the Porsche 911 GT3, any possible excess weight is removed to allow for the greatest acceleration, one that can compete with Lamborghinis and Ferraris.
Also, when people assume that more horsepower is equivalent to a greater top speed, they are ignoring a very important concept: terminal velocity. Yes, it is important to have a large enough engine to accelerate the car at a great rate, but ultimately when you start getting into the 200 mph area, design is much more important than power. As we learned earlier this year, air resistance is directly related to velocity. However, the extent of the air resistance varies greatly depending on the shape of an object (For example, dropping a book vs a sheet of paper). Therefore, it is necessary for car designers to minimize air resistance while still creating downforce in order to create the best results. The balance of these two goals and new advances in materials/aerodynamics is the reason that street legal cars have been increasing in performance greatly, but also the reason that creators are having a hard time creating a road legal car that hits above 250 mph, while still looking/feeling comfortable.