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).
Thanks for the info!
You're right, but I'm afraid it's far down the list of priorities. I could sell the mark II and recoup most of the cost of the new version, but I rather like the idea of keeping both, so it'll have to wait until after my next big lens purchase
5D mark III, 50D, 300D, EOS-M; Samyang 14mm f/2.8, 24-105L, MP-E, 85L II, 100L macro, 500L IS II, EF-M 18-55; 1.4xIII + 2xII extenders; 600EX-RT; EF-M--EF adaptor.
Former lenses include: 70-200L f/4 non-IS, 200L 2.8, 400L 5.6