Your poly count CHANGED? Sounds like part of the model isn't a vertex object at all. Either that, or you are working with subdivision surfaces; those automatically "collapse" when you export an object. N-gons are polygons with more than four sides. Some applications (like Poser) really go nuts when they see these. You can get away with n-gons for a while when you are using Carrara's subdivision surfaces, but it does not always triangulate them properly when you "collapse" down to simple geometry (aka, make it no longer a dynamic subdivision but now a mesh with a higher poly count). I've been doing the same mirroring you describe, deleting, duplicating, etc. (Also discovered that duplicating with symmetry in the Assembly room is a great way to work with symmetry in the vertex modeller). In only a few cases did I have reversed normals, and as long as I was careful to fix them before welding the halves, I was okay.