Add-on control is commonly used in control systems for disturbance rejection and servo enhancements. Despite the capability of greatly enhancing the steady-state performance, even an ideal add-on compensation can introduce significant output discontinuity and transient response. This paper presents an exact formula for computing the output discontinuity during add-on servo, and discusses its influence on transient performance in different control designs. We show that adding the compensation signal as an update of the reference yields much more desirable transient response than the traditional choice of add-on design at the input of the plant. Evaluation of the results is performed on a semiconductor manufacturing system.