Looking at the MTF you posted, it might be debatable how it would play out.
For example the 1.4 vs the native 800mm.
Notice the 600mm starts out higher, and maintains on the Sagittal, and it actually finishes higher on the outside.
The Meridional falls off on the 30mm line quicker for the 600mm.
I wonder how this will work out in the real world.

A description of the MTF's.
https://www.usa.canon.com/internet/p...ens-MTF-Charts

I often find myself shooting at F/4 over having a 1.4x just to get that extra stop of light.
But the big question, will any gain be worth $4,000.

The Canon RF800mm F5.6 L IS USM and Canon RF1200mm F8 L IS USM are both scheduled to be available in late May 2022 for an estimated retail price of $16,999.00 and $19,999.00, respectively**.