Both the ram air and brake cooling products should work well at high speeds. At lower speeds, they typically do not have access to enough input pressure. Bolt-ons also have a tendency to suffer from too many bends and ducts which are too small. The resulting turbulence cancels some of their benefit.
Ram air for the Viper intake can be made by extending the airbox forward and down, grabbing air coming through the grill before it reaches the radiator.
Archer sells a Viper brake cooling kit similar to the one shown in the Mustang post.