@User Such optimization strategies were researched a lot shortly after 130nm and dual Vt were introduced, i.e., at the beginning of the leakage scare. This paper discusses substitution algorithms: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.149.2983&rep=rep1&type=pdf . The leakage power efficiency gain they report is typical for these ideas (14%) although these folk manage to achieve 27% over a wide temperature range: https://www.researchgate.net/profile/RIris_Bahar/publication/220847432_Reducing_leakage_power_by_accounting_for_temperature_inversion_dependence_in_dual-Vt_synthesized_circuits/links/0deec52e123138c909000000.pdf