BIM's mirror effect is pretty cool. However, the main problem is that there is no player's body shown in the mirror. Also, it is very possible for the FPS to go down drastically. I have a very large level. In fact, that "thin white" line at the bottom of the screen goes all the way across the screen. There's good reason. My level takes more than 2 gb of memory. In fact it's 2800+ gb of memory. If BIM did not allow for an extra 2 gb to build (you have to turn the cap off) I would not be using it. I have already tried more than 3 gigs and it works so I imagine the cap is probably now at 4 gb. Back to the FPS. I put the mirror in this level and my FPS went from 50 to 1 7. Now that's pretty obvious.
THEORY - you know everything but nothing works. PRACTICE - when everything works but don't know why.
For me, theory and practice are combined: nothing works and I don't know why.