Yes I'm pretty sure the DIGIC 8 doesn't do anything to the RAW file, why is that a safe assumption? Because Canon only ever claim improvements on the OOC jpegs. I don't have a clue what criteria Canon have for setting the highest rating for the ISO, but 32000 - 40000 is 1/3 of a stop, hardly anything worth writing about.
Nevertheless I'd like to know where, exactly, people think they are seeing an improvement, because I have not seen anything that makes me think "a 5D MkIV couldn't do that". Not that I don't find may of the features of the R system interesting and improvements, just that IQ isn't one of them.
I very interesting comparison is going to be a 5D MkIV with EF 50 f1.2L and the R with the RF 50 f1.2L, as far as I can see Canon are saying the big IQ improvements are because of the optical freedom that extra 20mm gives them. Lenses are the difference, not the sensor or processor, for RAW files.
I don't think you're right in regard to this.
Firstly, the ISO setting in a camera obviously affects even the RAW file. So something within the camera is doing the boosting of the signal coming from the sensor, even when you shoot in RAW. If this weren't the case, the ISO setting you use wouldn't matter when you're shooting in RAW. But it still very much does.
Second, Canon does seem to call out that the processor in the camera plays into the image quality, even in relation to RAW. This is an old article, but still certainly relevant...
http://learn.usa.canon.com/resources/articles/2012/digic_processors.shtml
"What are the benefits? The combination of a 2-line 16-channel simultaneous signal readout from the CMOS sensor and the super-fast
14-bit analog-to-digital conversion (handled by four separate processing circuits, before the info reaches the Dual DIGIC 5+ processors) makes possible the following:
• Increase the sensor’s sensitivity by approximately two stops over previous models, meaning higher ISOs with the lowest noise of any EOS digital camera. The standard maximum ISO changes from ISO 12,800 on the previous EOS-1D Mark IV and ISO 1600 on the 1Ds Mark III up to ISO 51,200 on the EOS-1D X — without any ISO expansion applied."
So it seems that there's more going on upstream of the DIGIC processor that plays into RAW image quality. It doesn't seem like Canon fully specifies if it's upgraded from camera to camera. But if they're quoting a higher ISO range, I think something has changed.