Sadly the plugin is no better and I’ve had it disabled all this time.
The friend that I originally researched this all for reported, however, that Search Unleashed was not reporting by relevance after all. Since then I’ve seen details on Better Search, an alternative plugin that (supposedly) does provide relevance-based results. I’d try it out and report back back but after my initial activation, I deactivated it again. Rather than build upon your existing search template, it uses its own. So, by default, the search page doesn’t even look like your theme – you have to start the layout, etc, from scratch. That’s not something I’d like to do unless I really wanted to use the plugin. And I’m not that fussed.
But if anybody else does give it a go, please let me know!