Properly setup an SLI system will very nearly double (not quite 2x, but something like 1.95x) performance for each additional card you add. This gets to be fairly important for incident recreation cases where the court does not have the patience to wait for a render to finish. Enthusiast systems like we use for games max out at four cards, typically, which is more than enough for most games. The only place you run into trouble is where the user does not understand the memory limitations and they use up the benefits with extreme anti-aliasing (for instance).
I know firsthand what SLI can do, because I use it to produce animations regularly. Last quarter the other students were struggling to get renders done in the final week with the process running for days. When it got down to the two hour mark my instructor pointed out an issue with my rendering, and I had it rendered again and handed in within ten minutes time. There just isn't any comparison.
