Apple's fusion drive is not actually a single part; it's a combination of an mSATA SSD and a standard SATA HDD. The way fusion drive is implemented, both devices will need to show up with their own SATA interface for it to work.
Seagate does make single 'Hybrid Hard Drive' parts[1]: an HDD plus 8 or 16GB of flash to improve performance, but you won't see the performance gains that you would with a 128GB flash fusion setup. The flash cache is managed by the controller in those cases, so it's difficult to have apps or OS pinned there.
Alternatively, you can take out the DVD drive in your MBP and use an adaptor[2] to put in a SATA SSD. This would allow you to use Apple's fusion software. I've used this setup before, but its number one drawback is poor battery performance. Moving data lazily between HDD and SSD is fine in a desktop with continuous power, but could draw multiple watts of precious battery in a laptop. It turns out to be better to go full flash.
You have to be extremely careful with compatibility with these Optical Bay adapters and SSDs. I work at an SSD company and these are the #1 cause of issues in the field for users. While we don't officially support them, we still make sure we have functionality - which often requires special firmware customizations that we would not be making otherwise. I would also not expect any other manufacturers even test these.
What I can recommend though, is to go with either a Sandforce-based drive, any Samsung drive, or a Vertex 4.
Why? I also have one of these optibay's, and it doesn't contain any active parts: It's just a piece of metal to make your drive fit in the place of the CD drive. Why does this cause issues on the SSD's?
The same reason why putting SSDs in external enclosures is extremely inconsistent - power. An SSD needs significantly more power at certain times, depending on the controller. Most of the time, an optical bay does not provide the necessary amperage.
I think your facts are a bit off. The "superdrive" in macbooks require significantly more power than a SSD. When you burn a dvd using the superdrive, it requires between 40-50W of power. Even the most power hungry SSDs during heavy sequential write mode use less than 5W of power. If anything, the optical drive bay needs to provide more power than the HDD side of things.
It's more likely that the ebay-knockoff adapters were poorly made than it having anything to do with power requirements.
Oh, that is exactly what I meant - the Optical drive bays are all made very badly, and intended for HDDs. The motherboard/connection can certainly power any drive fine, but the replacement bay cannot.
Seagate does make single 'Hybrid Hard Drive' parts[1]: an HDD plus 8 or 16GB of flash to improve performance, but you won't see the performance gains that you would with a 128GB flash fusion setup. The flash cache is managed by the controller in those cases, so it's difficult to have apps or OS pinned there.
Alternatively, you can take out the DVD drive in your MBP and use an adaptor[2] to put in a SATA SSD. This would allow you to use Apple's fusion software. I've used this setup before, but its number one drawback is poor battery performance. Moving data lazily between HDD and SSD is fine in a desktop with continuous power, but could draw multiple watts of precious battery in a laptop. It turns out to be better to go full flash.
[1]http://www.seagate.com/internal-hard-drives/laptop-hard-driv...
[2]http://www.mcetech.com/optibay/ ($20 copycat on eBay works fine)