Interesting conversation here - but just another note to a few of you who missed my update, I did decide to just ADD a used 7D and keep the 5DII. I was more comfortable with spending that amount than than upgrading to the 5DIII, especially when the value of the 5DIII will probably drop faster over the next couple years than my older cameras.
Now, my issue wasn't that I could NEVER lock focus with the 5DII in low light or with movement, it just wasnt consistent enough for my liking. So, I agree - the 5DII CAN do it - just not every time. After a week or so of using the 7D here is what I've found - and probably not many surprises here:
-Outdoor moving subjects: The 7D with any AF point is generally more reliable than the 5DII is with the center point - although the 5DII center point is still decent. The 5DII outer points don't seem terribly reliable in comparison but can be used if you're willing to accept a lower ratio of keepers.
-For outdoor stills, the 5DII center point is about as accurate as any of the 7D AF points. The 5DII outer AF points lag behind but are still useable.
-Indoors and low light with still subjects, the 5DII fares better with the center AF point than any AF point on the 7D, but the 5DII outer points are very poor. All of the 7D's AF points seem to only slightly lag behind the 5DII center point.
-Neither camera is extremely good with moving subjects indoors but I can manage some keepers with both using center point.
All in all, I'm satisfied with the purchase as the 7D opens up some new opportunities for me but I don't think it necessarily solves my problem. But, between the two bodies I'll be set well enough that I can leverage the strengths of each under suitable circumstances and continue to use workarounds like prefocusing, flash focus assist, etc when conditions are less favorable.