[blue00gt]

The only thing that would make a tower bar slow you down is the weight, which is minimal. Having a stiff chassis is always good regardless of the car's purpose. A stiff chassis simply allows the rest of the suspension to do it's job the way it's supposed to without input from chassis flex. I'd say it probably has no noticeable effect going down the 1/4 mile.
The main reason to get a tower bar is the fact that on a Fox stang they have measured as much as 1/2" of deflection between the two towers under extremely hard cornering loads, which is not good for smooth, predictable handling.