The iMac FUSION storage drive OP mentioned is a hybrid drive - basically traditional HDD with flash memory bolted on for cache... similar to Seagate Momentus XT. It is nowhere near as fast as a real SSD and is limited by the speed of HDD, not by the interface.
The fusion drive is just that, a fusion between 2 drives. I have a 512GB SSD and a 4TB Hard Drive running in my iMac. The computer handles deciding which data goes where, and data that is accessed more often stays on the SSD. For smaller reads and writes it is as fast as the SSD. For bigger stuff, it slows to the speed of the HD. Considering it boots in about 20 seconds, and brings photoshop up in 1.5, AND has 4.5 TB of capacity, I am pretty happy with it. If one drive fails, you loos all data. So back it up...
Did you build your own Fusion Drive? It seems like you must have opened your iMac as that config isn't available as a BTO. I would caution others against this route, as:
a) It is currently untested, and slightly unstable. At the Apple Store we had some serious grief maintaining even Apple supported implementations of Fusion Drive.
b) You have to cut the adhesive holding the display in place.
c) Totally voids any semblance of AppleCare you ever had.