I would not use the technique of polygon > mirror. Best way is to freeze transformations and then move the pivot point to the centre of object. Duplicate it and then scale to -1 in X, then combine and merge.
Chris (formerly R@nSiD)
Twitter
When the power of love overcomes the love of power the world will truely know peace - Jimmy Hendrix
Winner SM VFX Challenge 1
3rd Place SM SteamPunk Challenge (May 2007)