Interesting! I have a 1.4xIII and a 2xII. Does using an extension tube make any difference to function, e.g. losing infinity focus?
Yes, it does mean loss of infinity focus, although you can still focus out quite a distance.
Since you have a 500 II, the 2xIII would be a good addition (except for the no stacking part). AF performance is better with the 2xIII, and the optical performance is better, too (the reason the 1.4x won't fit in the back of the 2xIII is that it has 2 extra elements compared to the 2xII (9 vs. 7).