The ACR package is realistically just a nice collection of bolt-on bits to the regular Viper.
I cannot see anything but an improvement in the car's overall performance by adding more track focused suspension, grippier brakes/tires and adding the aero bits to the convertible. Dodge already has an aero package for the convertible Viper. If the big wing out back somehow negatively affected the car's handling balance (which I really doubt) then they could just use the milder convertible aero bits, but still offer the ACR package with the rest of the track focused componenets.
I'm sure that the aerodynamics and balance of the Ferrari F430 Scuderia was also changed when they made a convertible version of the car, but it's no doubt faster than the standard F430 cabrio, as would be the ACR Viper roadster. The only factory addition that I would make, would be to add a
real rollbar to replace the cosmetic roll hoops on the convertible.
The ACR roadster would NOT be replacing the regular ACR Viper coupe, it would be adding to the Viper line-up and provide a top model convertible which would renew interest in the Viper, get it back in the eyes of the press, and ultimately compare favorably to many of the new high performance, two-seat convertibles out on the market today.