the xbox ide cable is non standard compaired to a pc ide cable as the xbox hdd and dvd plugs are just paralell extention's of the mother board plug.
on the pc the cable select pin is only active on the master plug for example and specific pins are joined on the host to show an 80 core cable is installed etc...
because of this it is not known if it may cause issuies because of legacy signals and missing pins/signals etc with the xbox...
there is also the issue that the pc cables have the cdrrom futher away from the host than the xbox and it is hard to find on that will be the correct length because of this some mount the master socket in the motherboard and the host socked in the hdd but this cause cause problems with cable select and you might have to force your hdd to master also it is shorting the ata mode signal at the drive which may cause issuies who knows?.
if you want to make a proper xbox 80 core ide cable it would be best to butcher 3 80 core pc ide cables pinching the master device plugs off them and installing them on a new lenght of 80 core ribbon cable cable to ensure 100% compatibility as the master device does not have any keying pins or misising pins like the slave and host plugs.
The xbox does not switch to ata 166 if you put in a 80 core ide cable i measured the speed this my self with a scope as i have been trying to figure out why you cant boot an xbox with a sata hdd with no dvd board.
all in all if you put an 80 core ide cable in it may work it may not its your choice but dont expect it to go alot better

hope this clears things up a bit.