It's not just the software stretch and loss of resolution, but the way this interacts with the noise pattern. Noise from sensor gain at high ISO should be uniform and consistent across the whole image, however once we change the geometry, we will be squeezing in and stretching out the noise, creating artifacts that look like a lattice, moire and so on. Furthermore, this conflicts with noise reduction principles that assume a consistent noise pattern across the image. This is further compounded b stronger vignetting on some of the RF lenses; I don't mind the vignetting look sometimes, but I don't often get to choose anymore due to these two features conflicting; that's around two stops on the RF 35mm f/1.4L VCM, so you've now got an image with ISO 51200 exposure in the corners of an ISO 12800 image, which in darker situations, results in near-unusable images if any corrections are made - and by my testing, the vignetting falloff occurs closer to the centre of the frame than it does on the EF variant. So whilst the t-stop is measured from the centre of the image in both cases, we ned up a lower potential average t-stop on the newer lenses (using this to account for transmission loss, which impacts low light performance at the same aperture).
You forgot Pentax
% but I've never thought to articulate it in this way. I also often need to shoot at higher ISO when outside due to a combination of field conditions and shutter speed requirements affecting available light. My specific EF zooms and primes, for all of their shortcomings vs. newer editions, do not suffer in this regard.
