The C5R's didn't really have a chance until they added the 7 liter engine which was against the rules if memory serves as no C5 Corvette could be bought with a 7 liter motor at the dealership. Also the Vipers were heavily restricted to make them eligible for the GT1 class and yet still dominated Le Mans and most of the other series.
Yeah Snakebitten,but Viper's still had 8.0L 709hp/729tq 2535lbs,compared to the Corvette's 7.0L 600hp/495tq 2511lbs. That's a bid difference,in racing. Viper just over powered the competition.