To the best of my knowledge, using mirror geometry will have little/no impact on the rendering, because the rendering is caculating based on the teselated geometry in the scene. When you mirror the geometry, you are creating a surface thats a copy of the original. it's no different then if you created the copy from skratch.

It is however, easier to model (IMHO) half a model then mirror it (I'm a fan of the CSP tool that connect poly shape tool (i think it's version three thats most stable), as it makes a smoothed proxy already mirrored so you can see the outcome of your tweaks on the low geometry mesh. you should be able to google it)