Female to Female Thunderbolt Coupler

The eMac is no slouch. And FireWire 800 can produce "near Hard Drive" speeds, FireWire 400 is a little slower. If the rest of your network is Gigabit Ethernet, you will see only a slight bottleneck at the Firewire connection.

I suggest you try it and see if you can live with the results. I expect you will be pleasantly surprised.

My plan is to go from firewire to thunderbolt

Sorry, No.

To run ThunderBolt, a computer must have been manufactured with ThunderBolt Hardware already installed. There is no ThunderBolt add-on available, and there may never be one.

The adapter you linked to (from FireWire to an 8-pin connector) makes no promise of using a standard Ethernet protocol. Its only promise is that if you plug in its mate, it will re-create fireWire on the other side.

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A simple solution to this problem is to connect the storage box to a computer with a FireWire cable, and use Mac File Sharing to access it from all your other computers. The disadvantage is that it must be on. If you have a recent Apple Base Station or Apple_TV, you can use "Wake on network Access" and allow the computer with the storage box to sleep.

Connect the G-RAID DIRECTLY to another computer with the fastest Interface it has (FireWire 800 or eSATA, for example) -- not to an Apple Extreme Base station with adapters.

Connect your local computers with Gigabit Ethernet cables (all eight wires connected), and set them for Gigabit Ethernet Speed, Full Duplex, flow control, and Jumbo Frames.

Then enable

System preferences > Sharing > File Sharing ...

... on the computer holding the G-Drive. [Sharing in Apple's view, means "Making available for others to use". Once available, you just use the Shared item (and that use is not referred to as Sharing.]

File Transfers go directly from Computer-to-Computer, they do not use the Router as intermediary. So they will be at Gigbit speeds.

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If you are concerned with leaving the computer with the G-Drive on all the time, set it to "Wake on Network Access", since it is supported (in a fairly round about fashion) by having a smart Apple Router or Apple-TV on the network.