A method of modifying a VLSI layout for performance optimization includes defining a revised set of ground rules for a plurality of original device shapes to be modified and flattening the plurality of original device shapes to a prime cell. A layout optimization operation is performed on the flattened device shapes, based on the revised set of ground rules, so as to create a plurality of revised device shapes. An overlay cell is then created from a difference between the revised device shapes and the original device shapes.