It would allow him to officially confirm whether or not his system runs slowly on a fresh install with nothing else on the hard drive but the operating system.
Of doubtful value. An archive & install bypasses any kernel extensions or drivers which may have been installed by third-party software. If software hasn't installed drivers or kernel extensions then that software can't affect the functioning of the OS. The only exception of which I am aware is startup items, which can be addressed easily without an OS reinstall. So for all intents and purposes, he's already done a "fresh" install.