I was testing the D700 today against the D2x with my Sig 500mm f4.5 plus 1.4x tc, now before anyone says, yes i know they shouldnt work, but with taped pins, it works fine on my D2x and my D40! On the D2x it was about half a second to lock target, and the D700 was like two or three! It was just chattering back and forth then would lock then loose it. Its fine without the TC smooth and predictable. Its going to be a bummer if this is an issue. I need the TC to make up for the lack of crop on some subjects! Now i havent tried this on any nikon lenses and TC's (i dont have a nikon TC to test with).