There is of course an optimum length for the tail pipe to give the 'perfect' power curve, but that's not to say that it will be ruined by a few changes.

If you're thinking of just extending the original tail pipes with the same diameter tube then be sure to jet up a tad for initial testing as you could suffer over heating and detonation. The best plan might be to cut the pipe right from the end of the final cone and then use tube a fraction bigger in internal diameter. with the additional length it 'should' act in a similar way to the stock pipe.