Congrats on the lens!

RE extender performance, I tested the EF MkIII and the RF extenders head to head on my EF 600/4 II.

https://www.canonrumors.com/forum/th...l-is-ii.41215/

I found the 1.4x versions functionally equivalent. For the 2x, in my hands the EF III was sharper than the RF. The RF 2x was actually similar to the two 1.4x TCs stacked. The RF also added noticeable barrel distortion.

For bags, I use a LowePro Lens Trekker 600 AW II (there’s now a vIII, I don’t know what changed). It does a great job, it’s basically dedicated to the 600/4 with no space inside for other lenses, but there is sufficient space for the mounted 2x TC + 1D X. A tripod attaches to one side, the attachment is robust enough for my RRS TVC-33. There are strap attachment points for LowePro’s lens cases on the sides as well. The only one I usually use is the Lens Case 1N, which they unfortunately discontinued a long time back, when they switched from their relative size numbering with a letter suffix (like 1N) to the metric measurements (e.g., Lens Case 11x14). In that transition several sizes were lost, notably the 1W which is perfect for many ‘standard-sized’ lenses with a hood reversed, like the 24-105/4, 16-35/2.8 or /4, TS-E 17 or 24, etc. Also lost was the 1N, which was designed specifically to hold both the 1.4x and 2x TCs, and even came with a little padded disc to go between them.

Enjoy the new lens!!