I'm curious about why more people don't stick the SSD in an external Firewire 800, Thunderbolt, or even USB3 enclosure? I've seen Firewire 800 with extra ports for daisy chaining for around $50, so only a little north of the data doubler cost. I understand that it's a tad slower than the internal interface, but SO much easier to install. It's my understanding that you can even do the fusion drive with the external if you really wanted to (though this would make ME very nervous). Booting via Firewire 800 is no problem.
FW is too slow, might as well get a hard drive. TB and USB have too few ports. Since there is an empty drive bay inside why not use it?