The study aims to furnish insights on the use of new impact indexes for ex-post evaluations in the framework of the post-2020 CAP. In particular, concerning water policies, the use of economic efficiency is considered and applied to three irrigation supply systems in winegrowing farms of northern Apulia, southern Italy. Results make it possible to investigate whether and how different irrigation supply systems interact with the management of productive factors and inputs, so as to understand complex and wider impacts of policies on costs and revenues of farms in the short and long run. Economic efficiency, used as impact index, can provide useful information for assessing the multiannual performance in several post-2020 CAP contexts and in connection to the related specific objectives. In particular, this index could be applied to assess the policy performance on farm scale in connection to the supporting of viable farm income; to the enhancement of market orientation and competitiveness; to the improvement of farmers’ position in the value chain; to the adoption of climate change mitigation and adaptation measures; to the preservation of natural resources; to the employment of young farmers; to the development of rural areas; to the improvement of agricultural response to community demands on food, health and animal welfare.