In this paper we consider the problem of combining calibrated performance models of system components in order to predict overall system performance. We focus on open workload system models, in which, under certain conditions, obtaining and validating the overall system performance measures can be a simple application of Little’s law. We discuss the conditions of applicability of such a simple validation methodology, including examples of successful application, as well as examples where this approach fails. Additionally, we propose to analyze the deviations between the model predictions and system measurements, so as to decide if they correspond to “measurement noise” or if an important system component has not been correctly represented. This approach can be used as an aid in the design of validated system performance models.