The Viper was wearing a set of Michelin Pilot Sport Cup tires, not your run of the mill street tire but far from the capabilities of a full racing slick.
To put it in perspective, a heat cycled out set of Hoosier Koni Challenge slicks, which are not an all out super aggressive tire to begin with, were worth about two seconds a lap over a fresh set of Bridgestone RE-01Rs, a pretty series street tire, on my RX7 at Nelson Ledges. This is on an 80 second lap so we're looking at a 2.5 percent gain in lap times from a crap slick to a great street tire.