1. A method of generating candidate cell specification data for one or more cells of a system design, the method comprising: receiving system architecture information pertaining to the system design; receiving system specification information pertaining to the system design; selecting a first set of cell specification data for the one or more cells of the system design; simulating the one or more cells of the system design with the first set of cell specification data to generate a first set of corresponding system performance outputs; building a prediction-based performance model of the system design from the first set of cell specification data and the first set of corresponding system performance outputs; generating a second set of cell specification data using the prediction-based performance model; and simulating the second set of cell specification data to generate a second set of corresponding system performance outputs.