I have no separeted account. And the mac that I use for playing live is my everyday mac! I just have 8GB of RAM, and had installed a 7200rpm disc. No WIFI while playing, And playing always on full screen or performance mode (these modes disable Time Machine, as stated here: http://help.apple.com/mainstage/mac/2.2/en/mainstage/usermanual/index.html#chapter=2%26section=4%26tasks=true ).
To reduce the load of your concert, use the copy as alias wisely. I never had issues with mainstage itself, sometimes, I had some issues with third part plugins. But mainstage, to me, works pretty well.