1. A method of operating a computer system to support evaluation of a user-visible feature of a graphical user interface provided by an application program of the computer system, comprising: instantiating a graphical user interface portion of the application program as a set of modules including a core module providing core functionality and a core module interface for interoperating with a variable module to be included in the set; selecting one of a set of distinct versions of the variable module and including it in the set of modules to interoperate with the core module using the core module interface, the distinct versions differing by respective implementations of the user-visible feature; and monitoring, recording and reporting usage of the graphical user interface involving the user-visible feature, the recording including collection of performance information for aspects of operation potentially affected by the use of the selected version rather than other versions of the variable module, the reporting including transmission of the performance information to an evaluation system that (1) collects the performance information from subject computer systems of a sample population, the subject computer systems collectively using all the distinct versions of the variable module and providing respective performance information therefor, and (2) applies statistical analysis to the collected performance information to yield statistical performance measures for the different implementations of the user-visible feature across the distinct versions of the variable module.