The Viper is said to have the Hennessey 700R package which is available for the fourth generation 8.4 liter V10 Viper SRT10. The dynograph of the package below shows 613 horsepower to the wheels. This roughly 100 wheel horsepower more than an R8 V10 with a tune and exhaust is capable of depending of course on the fuel, exhaust system, and which tuner. Fabspeed showed 505 wheel horsepower with a tune and exhaust on a Dynojet.
The video has great picture quality but the runs are not very well done. In the first race the R8 gets a huge jump and the Viper doesn't go after it. The second run is the best as the cars both get a decent start but the Viper quickly shows its dominance and let's off. It sure looks like a difference of about 100 wheel horsepower between the two.