In this paper, we study system performance in passivity based control designs. In particular, we consider the passivation method of [1], we extend it so to use non-positive passivation parameters and transfer functions, and present results to passivate systems that are not finite-gain stable. The passivation parameters are selected according to non-derivative based optimization methods when the system dynamics are unknown. We show that by tuning the passivation parameters, desired system performance can be achieved in addition to guaranteeing passivity for the system. To validate our results, we present simulations in CarSim/Simulink.