Originally Posted by
iomatic
...I'd suggest a Sigma or Ricoh GR-D II with tele adapter instead, otherwise as mentioned, a flash won't do any good from such a distance (oh the horror of on-camera flash). If you want to take it another step, try off camera, remote-triggered flash @ strobist.com. Join the flickr group and post questions, and find out more information.
You did note that the OP wanted to quietly take unobtrusive photos of theatrical productions?
Any kind of flash is absolutely a non-starter.