Indeed, I've been doing some digging this afternoon to try and track down some answers.
Perhaps the most concrete information I found was
this video benchmark done by OWC. It compares their SSD in a variety of MacBooks, but the most instructive comparison for our purposes is the Core 2 Duo MacBook Pro with SATA II vs. the old Core Duo MBP with SATA I, which takes over 10 seconds longer to run through the boot/app-launch sequence.
Unfortunately since the rest of the specs (RAM/CPU) are different, a direct comparison would be unfounded, but it certainly seems as if the SSD is taking advantage of the faster 300MB/s interface in the Core 2 Duo machine. If this suggestion is accurate, it would follow that one would also notice a real, visual improvement in a SATA II vs. FW800 scenario. Of course, once you get the OWC in your Mini, you'll have some first-hand experience with what the difference is actually like in normal, day-to-day use.
Elsewhere, OWC actually sells a FW800 external solution that appears to offer a noticeable improvement over both desktop and laptop hard drives. Aside from
OWC's claims (comment #18), I've found
two reports that attest to the performance boost, so you're not alone. Along those lines, one of my thoughts earlier today was that, hey, if you can get most of the benefits through a FW800 bus, why not just get a cheaper SSD that's not rated as fast (anything above ~80MB/s would do), and use that? Well,
one article actually simulated several months' worth of everyday usage on several brands of SSD, and it turns out all except the OWC suffered moderate to severe degradation in speed over time. So it looks like I'll have to keep saving my pennies after all....