I can only tell the difference between the 99 and the 11880, which have the same printhead technology and inkset, except for O+G.
And it's very subtle. You'll get smoother reds, skin tones and foliage on the 99. On a media to take it. E.g. glossy papers like the baryta/fiber. On matte papers there's not that obvious.
The CMYKOG is also a great advantage for spotcolors and proofing.
/Sven