If the 8K codec is H.264 matching the bitrate of the GH5's H.264 8bit codec, it'd be 60-90 MBps. If it's an inefficient motion jpeg matching the rate for 4K on the 5D Mark IV, it'd look like 250 MBps. It's important to note that I'm expressing the codecs in MBps and not Mbps.
If the camera can't run H.264 in 8K(that's a crap ton to process), then CF Expres would be necessary since it'll likely output in an inefficient codec.
And you're correct that the slower card can feature limit someone. LIkely more buffer for those shooting stills. For video people, the lower codecs will be fine initially. When the computers improve(mainly the video editing software), the camera will be ready for more video people to take advantage of it with their camera. I'm excited for the features the R5 will have, even if I won't use them for the first few years, just because it means I don't have to worry about the camera being obsolete. It's the reason I tried holding off on buying a R, the video is already about obsolete.