1. A design optimization method for reducing leakage power for a target circuit design specified by a machine-readable file, comprising: providing a computer-implemented model of timing constrained, leakage power reduction achievable by applying gate length modifications to cells in a circuit design as a function of a set of characteristics of circuit designs, the circuit designs including respective pluralities of circuit paths, wherein said set of characteristics includes starting slack, logic depth, power sensitivity to gate length modification and delay sensitivity to said gate length modification; determining values of said set of characteristics for a target circuit design; applying the computer-implemented model using said values to produce an indication of susceptibility of the target circuit design to optimization by a procedure to modify said target circuit design using said gate length modifications; and determining whether to execute the procedure to implement gate length modifications in the target circuit design, in response to the indication.