Most of the spraying is in the Street Tire classes, like ST/STS/STX etc. True "R" Comps work better when hot--and most autocross situations will not 'overheat' an R Comp.
It is not uncommon to see R comp shod cars with tire blankets (look like foil) on their cars between runs to keep heat in...
Street Tires, on the other hand, like the RT615, Star Spec, XS, RE11, RS3 etc. can overheat past their sweet spot--and is especially true on 2-driver cars and/or hot, sunny weather. The watering is on the tread face, more than the sidewall. Pump-up Garden sprayers work very well for this.