well, no, and yes
The interface is slower, but the drive might be faster. It really depends on the drive in your computer. If it is a laptop, an external 7200 rpm drive will probably be faster, but otherwise, no, your internal drive will be faster due to the peak transfer capabilities of the drive interconnect.
Firewire 800 has a peak transfer of 100 MB/s, sata either has 150 MB/s or 300 MB/s, so the firewire interface is slower, but the drive speed is what limits all of these technologies right now.
And, yes, you can boot from the external drive.