It is irrelevant when you argue that the stock Viper has a 650 BHP V10, and that it's the reason why the 4-cylinder Megane shouldn't be able to overtake it going up a hill. Given that the Viper Gr.4 has more power than the Megane Gr.4 it's actually fair that the Viper has a slightly worse power-to-weight ratio, since the additional power gives it an advantage when it comes to fighting the forces of aerodynamic drag. From that perspective the Megane should have better acceleration at low speeds, since that gives both cars a strength and a weakness. The importance of these strengths/weaknesses varies depending on the type of track. On some tracks the speed stays fairly low, and then the Megane has clear advantage. On other tracks, where the speed is higher, the Viper will have the edge. Circuit de la Sarthe - although it's not in the game yet - is a perfect example of a track where the Viper would probably outrun the Megane.
It depends on how much faster they are over a race, and if they're consistently faster on all tracks, or just on some tracks. Performance is usually not identical in real life racing either, you just have to work with whatever strength and weaknesses your car happens to have. And if some cars are consistently faster on most tracks they will probably have their BoP adjusted, sooner or later.