Since implementing this as well as the fail2ban blocking, I have reduced the number of spam messages that postgrey has to deal with by about 100,000 messages a day.
on 11/6 my daily mail log statistics that are emailed to me said
149622 rejected (96%)
on 11/8 it went down to (I think that is after I did fail2ban)
23317 rejected (98%)
on 11/15 it is now at
4727 rejected (95%)
So the combination of fail2ban, postgrey and country blocking has made a huge difference in performance.