The folks at Bare Feats have put together a handy look at the performance of four of the five storage options available when custom-ordering a new 27-inch iMac through Apple. These include a 1 TB and 3 TB hard disk (a 7,200 RPM Seagate Barracuda), 768 GB solid state drive and 1 TB Fusion Drive. Their results with the latter are pretty impressive, but come with a noteworthy caveat.
In their testing, Bare Feet found that the SSD option obviously performs best, offering a throughput of 440 MB/s when writing large files and 477 MB/s when reading large files. Small random transfers hit 208 MB/s when writing and 140 MB/s when reading. By comparison, the Fusion Drive managed 325 MB/s when writing large files and 482 MB/s when reading them (127 MB/s and 129 MB/s when writing/reading small files, respectively). The Fusion Drive saw a substantial (nearly 50 percent) decrease in transfer speed when attempting to move files larger than the free space on the drive's "fused" SSD portion, however.
As for the traditional hard disk options, they performed about as well as you'd expect. You can check out the full benchmarks on Bare Feats -- a recommended read for anyone thinking of going built-to-order.