I think the roads DO make a difference in what kind of engine is in a car, and how it handles, etc. If your only concern is drag racing, you need torque! Drag racing is about being the quickest, not the fastest. In Europe, there's probably more popularity with road racing/circuits, which is totally different from drag racing.
BTW, were you referring to the Viper's 8.0 liter V10 (510 horsepower for the most recent vipers I think, not 450) and the BMW's 5.0 liter V10? I think the Bimmer's V10 gets 18MPG on highway...which isn't exactly "fuel efficient". The viper is probably worse, but not by a whole lot when you consider 510 horsepower.
Lastly, as a "rebuttal" about technology in each respective engine; the BMW engine probably costs much more than the Viper engine, and the Viper engine most likely has PLENTY of room left for development (more potential for refinement). The BMW engine probably doesn't have as much room left for improvement. Eventually, they'll have to up the displacement, or pour more technology into the engine, which means MORE money.
Just my two cents though.