The performance that can be achieved in a power plant obtained upgrading a typical aero-derivative gas turbine is analysed. The methodology is based on the off-design analysis of the gas generator (compressor and high pressure turbine) in the upgraded plant configuration and is applied to the design of a power plant based on the recuperative water injected cycle. The gas generator operating region and its boundary have been evaluated for the upgraded plant configuration; an optimization procedure has been established in order to show the maximum efficiency and power output that can be achieved.