I'm not entirely sure what the M5 owner was thinking, or even if he was thinking at all. I own both cars, and the Viper is clearly much faster. There's no special magic to it -- the M5 weighs 3975 lbs and makes 330 RWHP, the Viper weighs 3450 lbs and makes 405 RWHP. The M5 is a wonderful car, and is very high-performance for a 2-ton luxury sedan, but it's not a race car.
FWIW, my best 1/4 in my M5 were 13.5 @ 106 mph, which is a far cry from even a stock Viper's 12.3 @ 117.