the difference won't be noticable at all.
booting from a FW is possible, and even though the RPM will be faster on the External, you still deal with the bottleneck of FW. its only so fast. i booted off of an External drive for about a year with my 12" powerbook, it wasn't any slower or faster in real life usage. FW is faster than USB would ever be, sure, but you won't be able to gain anything out of it even with a faster drive because its still slower than the standard hard drive connection. you might notice a small speed increase if you were to put a faster drive into the powerbook though.