The speed you are seeing is reasonably typical of what I see using a self-powered OWC firewire enclosure with a 350 GB 2.5" 5,400 rpm drive salvaged from an old PowerBook.
The electrical properties of a poor quality cable may result in an increased transmission error rate which in turn makes it necessary to resend data packets which in turn reduces the effective transmission rate. However, the data transfer rate when creating a clone is a function of a whole lot of factors most of which are completely unrelated to and unaffected by the thickness or relative quality of the firewire cable. Regardless, I am a firm believer in using the best quality firewire and/or USB cables I can get and I have seen some really crummy cables that were pretty thick.
IMHO the relative diameter of the cable is not a reliable indicator of the relative quality of the cable. The most consistently reliable indicator I have found turns out to be price. In other words you get what you pay for. My personal choice of firewire cables comes from
Granite Digital and they offer a 7 inch, 9 pin to 9 pin, firewire 400 cable for $15.95 + shipping and handling. I use these when I have daisy chained firewire 400 devices and I have been extremely well satisfied with their performance for years. FWIW I believe you are definitely on the right track in looking for a shorter cable length. The longer the cable the greater the opportunity for cable resistance and capacitance to degrade the signal not to mention the greater the risk from outside electrical interference to get into the signal.
If you want to speed up the creation of a clone some suggestions are:
- Use firewire 800 instead of 400 if possible
- Use a faster computer with more memory
- use faster hard drives on both ends
- before beginning the clone
- clean out all the cache and log files before starting
- use Diskwarrior or Techtool Pro to optimize the directory structure
- optimize (defragment) the files on the source drive
- optimize (defragment) the source disk
