Update!

The original SIIG NN-E38012-S1 card referenced below stopped being recognized by my computer at boot, so I exchanged it for an identical card. The replacement card is much faster than the original, and — while still not quite as fast as the built-in FireWire — it comes close.

The below comparison references the SIIG NN-E38012-SA that eventually failed. As of 28 May 2007, the replacement card is still going strong.

Comparison between:

• Built-in FireWire 800 on the dual-core Power Mac G5, 2.3 GHz
• SIIG NN-E38012-S1 PCIe FireWire 800 Triple Interface, FireWire 800 Port (which later proved to be defective)

Notes:

• My LaCie d2 Extreme 250 GB drive has mediocre random read/write performance. The only meaningful tests are the sequential tests.
• The 4K block test routine of Xbench tests very low on all disks. The 256K block test more closely corresponds to actual throughput via other applications.
  
  
• There is a thread going on regarding this issue at the Apple Discussion Forums. This post is of particular interest:
  
  

Re: Slower FireWire Throughput on PCIe Card

From rob_ART; Posted: Feb 12, 2006 9:52 PM
in response to: SkyNite

I tested the Aaxeon FW800 PCIe card on the Quad-Core.
I'm only getting 24 MB/sec write speed.

They said the TI bridge chip has a design flaw. Hopefully
it will be fixed in the next run -- around April.
The Aaxeon USB 2.0 card is faster than the Quad-Core's built-in
USB 2.0 ports -- at least on READ speed.

My Test Results Using Built-in FireWire 800, no other devices except for LaCie d2 Extreme 250 GB FW Disk:

My Test Results Using SIIG NN-E38012-S1 PCIe FireWire 800, no other devices except for LaCie d2 Extreme 250 GB FW Disk:

Other User’s Test Results Using Built-in FireWire 800:

Other User’s Test Results Using SIIG NN-E38012-S1 PCIe FireWire 800:

My Retrospect Test Results: