The RF 1.4xTC works well on my 100-500mm and R5 with excellent AF and after some sharpening at 700mm it's pretty close to the RF 800mm in resolution and IQ. I am intrigued as to how well the RF 2x performs in terms of AF and IQ on the 100-500mm in particular. It's so expensive, that it would cost not much more to buy an 800mm RF. But, if the IQ etc are good, it would be tempting to have one in my kit for the occasional 1000mm f/14 shot. I'd be grateful for any first hand experience or for pointers to any reviews (TDP has image quality with it only on the 800mm).