Xbee in cyclic sleeping mode at low and high temperatures

I have a two XbeePro setup for testing cyclic sleeping mode (8) for both. It must work at low (-40) and high (+85 Celsius) temperatures. What is then the maximum sleeping time and minimum awake time? Can I add an external accurate, temperature compensated crystal oscillator to improve it (and thus reduce power consumption)?

Unless you use the Pin sleep option, you are not able to add an external crystal.