1. Don't RAID 0 SSDs.
2. Your bottleneck is the firewire port, even for a single SSD:http://macperformanceguide.com/Storage-Drive-SATA-vs-Firewire.html
It is most likely pointless to put those SSDs behind your Firewire port. It is like getting a Ferrari and then speed limiting it to 45MPH. Could you do it, sure. But why?
For offloading video in your setup get the largest disk you can that more closely matches the throughput of the port. You will get more space and realize the same performance (except perhaps for random seek times, which may come back a bit quicker).