I think this would be a good change that could bring Viper into the meta, and make her somewhat useful. The same way that you hear a singeing sound when an enemy player (or teammate for that matter) steps into a Phoenix fire wall, players should cough when passing through Viper's smoke, even if they just enter it briefly. This would bring some new utility to Viper, and I believe would be balanced based on the fact that Viper's gas can't stay up for too long before needing to come down.