Test Plan for Performance related testcases

 You can prepare test cases for load testing as below ..
      Doc history
     General Testing Guidelines and Test Configuration   
      Load Test Strategy and Guidelines   
            - End-to-End load testing
            - load testing of each component
     Summary of Deliverables   
            - Application response time,tps in various load strategies
            - resouce utilization
            - tuning, comparison  
     Test Case LOE and Summary   
     Test Case Summary   
     Test Case Descriptions   
     Scalability and Sustainability  Tests   
     UI(website) Sustainability (concurrent sessions)   
     feature1 Throughput
     feature2 Throughput   
     Mixed Load Sustainability (feature1+featrure2+...+featureN)        
     Fault Tolerance