I don't have numbers to prove it, but my wife's MacMini certainly feels faster when booted from the external Firewire 800 drive than it does running from the internal drive.
That wouldn't surprise me. FW800 tops at 79MB/sec by my tests. One larger 3.5" sata I tested with esata only did 89MB/sec. When I wondered why I checked the specs, and that was that drive's theoretical performance max. Laptop drives are historically slower, so I could see an older 2.5" sata, or especially a 2.5" IDE, being a good deal slower than FW800 attaching a faster drive.
(btw, macs can also be booted off SD cards, esata, thunderbolt, and a variety of even less common things, such as compactflash)