1. A computer-implemented method of predicting an overall system performance and capacity of a target information technology (IT) system based on component-level performance statistics, said method comprising: a computing system obtaining reusable software modules from a library of reusable software modules; subsequent to said obtaining reusable software modules, said computing system performing test runs of said reusable software modules individually and performing a test run of customized middleware; based on said performing said test runs of said reusable software modules, said computing system determining sets of component-level performance statistics measuring individual performances of said reusable software modules in said test runs; based on said performing said test run of said customized middleware, said computing system determining middleware-based performance statistics that represent a contribution of said customized middleware to said overall system performance of said target IT system; said computing system storing said sets of component-level performance statistics in a database that associates said sets of component-level performance statistics with said reusable software modules; said computing system storing said middleware-based performance statistics in said database that further associates said middleware-based performance statistics with said customized middleware; subsequent to said storing, said computing system determining said target IT system requires said reusable software modules to be components of said target IT system, wherein said target IT system is not yet built and is different from said computing system; said computing system retrieving said sets of component-level performance statistics from said database based on said determining said target IT system requires said reusable software modules and based on said database associating said sets of component-level performance statistics with said required reusable software modules, wherein each set of component-level performance statistics characterizes a contribution of an associated reusable software module to said overall system performance of said target IT system; said computing system retrieving said middleware-based performance statistics from said database; and subsequent to said retrieving said sets of performance statistics and said retrieving said middleware-based performance statistics and prior to said target IT system being built, said computing system generating a prediction that includes computing resource requirements of said target IT system and further includes characteristics of said overall system performance of said target IT system, wherein said characteristics of said overall system performance include a transaction response time, an input/output (I/O) activity level, and a network message volume of said target IT system, wherein a result of said generating said prediction is a generation of a recommendation of computing resources that are required to build said target IT system without over-specifying said computing resources, and wherein said generating said prediction that includes said computing resource requirements of said target IT system is based on said sets of component-level performance statistics measuring said individual performances of said reusable software modules in said test runs of said reusable software modules and is further based on said middleware-based performance statistics that represent said contribution of said customized middleware to said overall system performance of said target IT system.