i did this same thing, only the other way around, i made mine longer. i used USB tho. i'm not sure about firewire as i dont have one laying around to cut open but it was pretty easy with USB. You should be fine cutting it. i haven't have any problems with the throughput speed on my USB cable either. just have to becareful, are firewire twisted pair like CAT5 or more like USB just twisted all together? good luck to you.