You need to tell us in more details what you are editing. FW800 can handle ProRes HD and DVCProHD just fine, with the FW800 you can get a couple of streams. If you are trying to do a lot of multicam where you are pulling a lot of streams at the same time you may want to change.
I like OWC a lot, buy all my RAM and HD from them. While you lose the apple warranty and it is covered by OWC, I fully trust them there is only one source for service work, just something to be aware of.
But I will say that an iMac with a eSata port is a different beast all together.