A supertelephoto designed from the ground-up for RF could have numerous improvements, such as being designed to take advantage of the 11 RF mount pins, which can transmit additional information to the body and potentially improve focus speed/accuracy.
The rumors for the RF supertelephotos are very exciting, including:
- A very small/lightweight 500mm F4
- A 300mm F2.8 that might have zoom capabilities
- A 200-400 or 200-500mm that might have a built-in TC
All of the EF equivalents for these lenses are older and due for a refresh. Even if the refresh advantages are not dependent on RF design (for instance, it might have been possible to make the lenses lighter in an EF mount, but Canon didn't yet), it will still be exciting to see these advantages materialize for the RF mount, which they have not so far.
Additionally, it is possible (not guaranteed) that these lenses will be less expensive than the 400mm F2.8 ($12000) and 600mm F4 ($13000).
There are lots of reasons to be excited about new RF telephotos. Yes, the 400mm F2.8 and 600mm F4 are now available in RF mount, and that is great news! But it's also nice to see what options will be made available in the future, for the people who don't necessarily want to buy those exact lenses.
The 500mm F4 II (Release in 2011, $9000), 300mm F2.8 II (Release in 2011, $6100), and 200-400mm F4 (Release in 2013, $11000) have not been refreshed in a very long time, so it is exciting to see what can be done for these lenses with 10 years of additional technological advancements.
Having more choices to choose from is great for consumers, as we each have different needs.