There isn't an 'optimal' value for Expertise. That thread has tests to prove that having 1340 Expertise is not a waste of stats and that the damage gain from swapping to power is negligible.
There are a lot of flaws in his experiment. It's a decent read though, and there are a lot of varied opinions on the whole subject. With 1200 expertise, 1034 power, 2006 AIM, 1714 end, my average IP / FB hits for 100 - 300 more than his (that's running 7/3/31). I haven't seen what it would be like w/maxed expertise, b/c I'm too lazy / cheap to do it.
With that amount of expertise, I still get smashed / hit for about the same.