+1 on the mirror pros.
I've cut both glass and mirrors in my checkered past, and it really only takes a slight error - a change in the cutter pressure, a wiggle in the score, etc. - to trash what is sure to be a fairly expensive piece of mirror. Plus you can ginsu yourself right into the ER very easily with a pane that size. When those things let loose, they don't always just crumble top the floor. Sometimes they literally explode and blow glass shards all over the place.
I've cut that stuff before...and I'd take it to a glass shop. I wouldn't do it myself, personally.
Yeah, I'm learning how truly difficult it is to do this...Especially when you add a couple of holes and right angles. I had enough trouble making the cut below tonight...and it's still not perfect. I did have a lot of mirror to practice on, but...I pretty much used it all for that one imperfect square. I may make one other square for the other side (and if I have enough, one more for this side) but I'll probably either go with plexi-mirror or pro cut mirror on the back wall. I really like the way it will look though.

SideMirror