Ok, understood now.D-Nice told me BFI is per picture mode.
So, you could switch off BFI for all Dolby Vision content (as it's a separate picture mode), in the same way you can switch off Smooth Gradation for example. However, it sounds like the BFI setting in Custom picture mode would apply to both HDR and SDR.
If that's correct, then it would be good if Sony would allow BFI to be set independently per picture mode depending on SDR or HDR, in the same way the brightness setting can be.
The issue is that 'Custom' is the most accurate mode for both SDR & HDR, hence BFI will be universal to all content being played in that mode.
What's the difference between 'Cinema' & 'Custom' picture mode? I wonder if one can be set with best SDR settings and the other with best 'HDR' settings and just manually flick between the two on a case by case basis?
I suspect this isn't advised because 'Custom' may be more accurate?