You are absolutely right.
Just that we call them wrongly ... we should call them 40-pin 40-wire cable and 40-pin 80-wire cables. But commonly ppl call them 40-pin and 80-pin cables.
Both type of cables dont have different sockets, only the thickness of wires looks different from outside.
(And these CD-Writers give only a 40-pin cable with them .. never seen one which gives 80-pin cable with them.)
The following pics will make it clear.

All sockets in a 40-pin cable are black in colour.
and sockets in 80-pin cable are blue, black and grey in colour (blue one goes to motherboard)
About permorfance ..
Yes .. there is difference
40-pin cables support a maximum of "Ultra Mode DMA Mode 2" (33MBps)
80-pin cables can reach "Ultra Mode DMA Mode 5" (100MBps)
(both are pretty high values .. doesnt affect day to day work .. but still better if we have 80-pin one)
Some real good info about this:
http://www.storagereview.com/guide20...modesUDMA.html
http://www.storagereview.com/guide20...confCable.html
http://www.storagereview.com/guide20...nfCable80.html
Next time you go to buy an IDE cable .. be sure to buy a 80-wire one .. it's worth the money.
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alibi