I recently picked up a Type III 2x converter, primarily for use with my 300/2.8. I tried it yesterday with my 500, and it looks like it works pretty darn good
I suspect the focus would be faster with a Type II super-tele but h*ll will freeze over before I can afford one of those! Between using a "bump focus" technique, and tracking a specific bird throughout a shot sequence, the Type III TC held its own nicely.
