You guys with 4 piece really should try out arcane again now. At 520 ilvl I'm going a full haste invocation build (courtesy of Mishh in the arcane thread) and the dmg is very, very good. I can get over 40% crit self buffed, but comparing fire and arcane, arcane wins for me because it's just so much more consistent; I just got fed up with the RNG of fire. I've played around with both ROP mastery and now full haste invo and the latter gives a lot of freedom while probably doing more damage to be honest.
You've got me interested. Can you give the link to that thread and/or give some insight as to how to play haste arcane this tier?