Mercedes just presented a GT3 version of their new Mercedes SLS AMG super car. The difference between the street and the race car is enormous and it's not just the big wing on the back of the car. Actually you wouldn't want to try the GT3 on normal roads. That's how far away the race car is from the street car. It wouldn't be legal also.
In the past this was very different. The street version of a Ford GT40 was very similar to the race version and actually most if not all GT40s were road registered.
And that's what I love about race cars from the past. They were pretty close to what you got from the manufacturer and the changes were marginal in comparison. Racing immediately influenced the production and made the street cars better. There's barely anything floating back from racing car engineering into series production anymore, not even for super and hyper cars. And that's a pity!