I have owned three ZR1's and two Vipers. My fastest ZR1, 1993, ran 12.8 at Desoto Raceway in about 65 degree weather with me driving. At the same strip, on a hotter day, I've run 12.4 in my 2000 Viper GTS. Both cars had mild exhaust mods only. That should answer the question.
riw :laugh...