In reply to Lynsety:
It will be fine - there's usually a good foot at least left over. You'll notice an improvement if your current cables are tired, but how much of an improvement you would notice with Jagwire I couldn't say I'm afraid. Check that the ends of the outer aren't corroded - you're wasting your time replacing the inner if the outer is past its best.
There was a thread on here recently about cutting cables without cable cutters. There are various options, but buy a pair of basic cable cutters and save yourself no end of grief.