I have an LG DVD writer which is capable of writing discs at 16x, but I've never seen it burn that fast.

I had a spare 80 wire cable lying around, so I swapped it for the 40 wire cable that was in there.

It made no difference at all. 8.5 minutes to burn a 4.7Gb DVD (on 16x media) with either cable, although the speeds fluctuated a bit more with the old 40 wire cable.

Does this sound right? Which cable should I really have in there?

Optical drives like a DVD drive has a limit of DMA-2. You will not get any speed boost using 80 wire/40 pin cable. The extra 40 wires is used to seperate the data and sync wires, so it minimizes crosstalks. Writting at 16X with a DVD discs adds a lot of distortion for each pit. I suggest writting at 1X speed to cut down pit distortion. Too much pit distortion can ruin data and other readers may not able to read it even though DVD discs includes ECC. I doubt the drive can write at 16X all the time. Manufactures mostly state peak instead of the average. Also 2 MB cache for a DVD drive is way too small. An 8 MB of cache should be used, but a few drives have this.

I suggest using the 40 wire/40 pin cable instead of wasting money using 80 wire/40 pin cable. I recommend using 80 wire/40 pin for hard drives because there is a 10 MB difference.