i dont think having more materials will increase render time.
after all, you haven't increased the amount of work that needs to be done since you are using the same texture just mirrored over. it's still going to go through the same calculations

about that definite line between your model... did you duplicate the half of the model, then do a negative scale, then do flip normals, then do merge verts along the border edges?
if you did then you missed out the softening of the centre edge that used to be the border edge. that may or may not be the problem. i would confirm it but this computer lacks maya...