The increase was uncovered by Road & Track courtesy of the SAE J1349 certification process to which Detroit's Big Three automakers submit themselves and which reports the Viper's output at 645 hp instead of the 640 it was rated at until now. There are a hundred factors that could have contributed to the relatively mild boost in output (best guess? nothing at all changed...), but we doubt anyone's going to complain about some extra horses under the hood.
The five-horsepower boost brings the Viper that much closer to the 650-hp Chevy Corvette Z06, not to mention the 707-hp Hellcat, but the Viper's impressive power-to-weight ratio ought to mean it'll have little problem keeping up in a straight line – which is just one of the reasons why Chrysler won't shoe-horn the Hellcat into the Viper: as R&T points out, the supercharged engine is too heavy and the blower makes it too tall to fit in the Viper's engine bay.
Chrysler's big-bore V10 engine has come a long way from the 400 horsepower it produced back in 1992, increasing displacement by only 24.5 cubic inches but power by over 60 percent over the past 22 years.