Thanks, Keith. Good to hear you're getting nice results with GFS on the 8300, so I know it's possible.
I did make a custom media setting, and have been using that. I note that in addition to PSG280 as the media type, I have Head Height set to "Low" and Maximum Ink Usage set to "Medium High".
I've scanned in a close-up of the background of a photo printed on Ilford GFS and also on some HP Photo Paper I had left over from my old HP Z3100. Neither of these papers have any stippling, so the difference you see is not due to paper texture. This is quite easily noticeable on the print without magnification.
I see that Canon released a new Media Information file earlier this month, so I'll download that and try to set up the custom media setting again. I'll also play around with other settings to see if I can get some improvement.