Trying to remember my physics lessons so many years ago...
I'm no physics expert but based on the ???th law of Newton when the weight of a car is increased you will need more energy (read: more hp) to get to the same speed. So increasing hp will have an effect on top speed. With diminishing returns though, because at higher speeds you'll need a bigger increase in hp to increase the max mph with just one mph (because the increase in drag is bigger from 150->151 mph when compared to 100->101 mph).You are correct, and our cars do not have a good CD. Regardless of the cd of our cars, there is one fatal flaw with my horsepower calculations based on what our cars weigh. That flaw is when you begin to talk about top speed. In order for our cars to have the same top speeds as cars such as the Porsche 997, then we really would need to have those kind of HP numbers. As far as I know (please correct me if I am wrong physics guys), the weight of a car really does not affect its top speed. The weight will affect the amount of time it takes a car to get to its top speed (in other words the acceleration), but it will not affect the top speed. The only way that you are going to increase the speed of a car is by changing its aerodynamics, changing its frontal surface area, changing its gearing, and finally, changing its horsepower.
Lets say that my 2,000 lb 220 hp Exige S has a top speed of 150 mph, and a 0-60 mph time of 4.5 seconds. If I reduce the weight of my car to 1,500 lbs, and I change absolutely nothing else (other than maybe the spring rates and tire pressure, so that it has exactly the same stance as it did before), then I would expect my top speed to still be 150 mph, but my 0 to 60 mph time should drop significantly to something like 3.5 seconds.
I did not bring up top speed because we don't live in Germany, so who really cares. Our cars will do about 150 mph, that is plenty for the U.S. as far as I am concerned.
Actually because of the hp deficit, which is calculated from torque; so you are right. But as I said above, our power to weight ratios allow us to have the performance of 300 hp + sports cars; but jut not the top speed. To achieve that, we would really need to up the hp.