Fusion Drive utilizes a component of OS X called CoreStorage to work. When you boot your Mac holding down Option, you are booting into a screen running in the low-level EFI environment, before OS X has started. EFI doesn't understand what a home-built Fusion Drive is, but sees two physical drives capable of booting into OS X.
I'm not sure of the mechanics of why factory-built Fusion Drives in the latest Macs act differently but my guess is that there is a firmware chip of some sort, like you said. Apple is known to put custom firmware on some of their hard drives.
In the end I went with the most expensive but also the fastest option: #1 which was the Mercury Accelsior PCIe based SSD hardware RAID array (RAID 0 by default which I will keep) in a Mercury Helios Thunderbolt PCIe chassis. The main reason was because it was the only one that mentioned it was bootable (and made a big deal out of that fact).
I paid $799 for the 240 GB model (it's 128 GB X 2 minus the unusable amount...a nice touch for the seller not to count that). It's got a lot of technology built into the card including processors and cache etc. to monitor and control the hardware RAID array including extra error checking and load leveling/balancing. Also it's theoretically upgradable (each drive is called a "blade" and while they don't yet sell independent blades they may in the future). Also the PCIe chassis is an extra bit of the investment that could live beyond the SSD, just like the whole arrangement could live beyond the use of my iMac itself.
This is a great review on it where they say it's "really fast. Ridiculously fast."
A guy on youtube has a (somewhat boring) video of his Mac Pro taking 3 minutes to boot and launch paint shop pro with his hard drive and then after setting up one of these it cut down to 15 seconds! Crazy balls out fast.
This is the sellers sites for;
Of note is that if they didn't offer the bundle discount and if it were not so much more expensive I might have opted for this much more attractive chassis.
Or for even more and larger chassis with multiple slots and supporting larger sized cards.
These chassis got me thinking about the possibilities with thunderbolt. Now any iMac or MacBook can be as capable and expandable as a Mac Pro...there are even developments with video cards in one of these things! I do wonder if it would be possible to take a MacBook Air and hook up a thunderbolt PCIe chassis with a high end graphics card with the display on the internal monitor?
If I was one of these manufacturers, or even Apple, I would consider making a full on chassis with PCIe slots and storage areas, etc. I really wanted a Mac Pro but they were so old...now I can get some of that goodness piecemeal, though for a price. :-)
I get the device about a week from now and will post back my impressions if anyone is interested.
Best Answer
Generally speaking I BELIEVE that RAID requires drives of equal size. In some cases when you get drives of unequal size the RAID uses the size of the smallest drive, so in your case you would end up with a 256GB (128 + half the 256) RAID-0 Drive.
HOWEVER I have not done such a thing on Apple hardware using disk utility, it MIGHT give you a RAID of 256 + 128 GB, but I kinda doubt it.
But in any case creation of the RAID erases the data on both drives so if you are OK with that, try setting it up and let us know the results.