Unless you are going with the 70-200mm F/2.8L II or one of the new SuperTele II's, stick with the II version of the extenders. I bought the III version when it came out and returned it. There is no benefit in buying the III version for the 135mm.

Every time I take my 70-200mm F/2.8 II on an outing I think "dang this thing is heavy"....when I get home and put my pic's on the computer I think "dang this lens takes good pictures".

My reasoning on the the 5D III is because I own the 1D IV and now how well the 1D's AF is supposed to be. Like you I want to see more reviews, specifically on the AF system, preferably by someone who has experience with a 1D AF system so that I can hear a fair comparison. I know how much my keeper rate improved with the 1D IV over the 5D II, and it is significant.