On an MBP with a hd connected via FW800, I loaded a 1.5gig file into memory (Photoshop) and saved it back to the FW800 drive.
I then connected an FW400 device on the end of the chain, after the FW800 drive, and repeated the test, and yes, the transfer times were longer, indicating that having the FW400 device at the end of the chain does indeed impact on the bus speeds. (Write speeds were nearly twice as long, for instance.)
Try a test yourself in various configurations, repeating each test a few times and averaging out the results. I chose to read into memory and write from memory to try and factor out additional drive speeds when copying files between drives, but like I say, it wasn't a particularly scientific test.