Plant length dependent performance is commonly observed when performing feedforward active noise control for broadband disturbances. This is primarily due to high levels of low-frequency noise and limited coherence between input and error sensors. These types of systems present trade-offs in choosing between performance, output power requirements, and plant length. An additional design opportunity exists in terms of balancing the levels of cancellation achieved over frequency. The selection of an error signal emphasis filter determining the frequency dependence of the cost function being minimized by the controller adaptation can be of critical importance. While several explanations for this type of behavior have been offered, little work has been presented for quantifying performance limitations. A method based on calculation of Lagrange multipliers is presented for determining the optimal power limited solution of an FIR filter-based controller. This provides a useful tool for predicting the effects of emphasis filter design, actuator limitations and plant length on performance. This prediction method and the associated trade-offs are illustrated using data from an HVAC fan.