I also tried intel igpu+6bit-frc external display over hdmi and I find out that when ditherig is on (all dithering disabled) that bands when watching gradient are not so much visible as compared to laptop panels.
What full model name was the external display?
It would be interesting to see the results of dithering.exe + igpu on a real 8-bit panel (like Gigabyte M32Q)...
Enabling color management (.icm color profile) also affects the smoothness of gradients - it worsens them. I have long come across an assumption that enabling the color profile automatically turns on dithering on ATI/AMD gpu drivers. In any case, it is better to check whether you are using color management, as this may further affect the smoothness of the gradients.