for firebirds, a good sun keeper+source beats out a good maximus pretty handily. But a good maximus is easier to get, and beats out a mediocre sun keeper. One thing to keep in mind when evaluating weapons for greater rift use is the way you play in greater rifts, which is generally herding white mobs into elite packs and trying to always be fighting elite packs. That's why furnace is crazy, in addition to the biggest firebird DoT you also can generally play rifts in such a way that you get full benefit on a 2H from the 40-50% elite damage bonus.
I hear you but the #1 Wizard on the EU leaderboard is using this setup.
http://eu.battle.net/d3/en/profile/Simba-2186/hero/47364702
That's solo GR40.
So while I respect that you like the Sunkeeper, the best Wizard in EU and most of the top Wizards in US last time I checked are using 2H weapons (specifically Furnace or Maximus).
It's just the truth of the matter. Now, whether this gentlemen and others would be doing higher GR's if they had a near perfect Sunkeeper + Source instead of their near perfect Maximus could be an interesting discussion...but considering the most common Blizzard + Hydra + BH builds do not benefit from the higher attack speed that 1H weapons offer, It's not suprising to me that big, badass 2H weapons are leading the way in Wizardville.
YMMV, but I'mma take my cues from the very best. If I get a godly Sunkeeper, I'll certainly keep it in the toolbox. But I'm socketing my Maximus and hoping to find a SoJ. With a 10% damage roll and a Gifted socket, my Maximus will be 3742. The equivalent of the #1 EU Wizard's weapon. Good enough for me. And again, if I get a decent Furnace by some miracle, I'll equip it instead. Until then, I have my goal.