Sharing our experience of cutting a mirror down as an improved build plate for our Replicator 2. Hope it’s helpful to anyone else thinking of doing the same!
Quite a painless process which will negate the need to get it done at a glass cutters, and you’ll have it in 5-10 minutes flat! 
More over at - http://funbiestudios.com/2013/03/mirror-build-plate-for-replicator-2/