Supercooling gas for more horsepower...

The theory of icing the intake was so it wouldn't heat soak the carb to 200 degrees between rounds. This was even more helpful in the days before air gap intakes. Cool can kept the carb from getting heat soaked during staging and burnout when there's not much cool air moving through the engine compartment, especially at tracks where you weren't allowed to open the hood in staging lanes. Hot restart problems from vapor lock in the staging lane is no good.

Not sure about the dry ice. It might be possible to over cool the fuel and cause problems with atomization. Also dry ice as it melts (sublimes) creates carbon dioxide. Not sure I'd want to risk building a CO2 cloud under the hood and possibly sucking it in at launch. They fill fire extinguishers with that stuff.