Hi Nil!
Flew about 150 similar sectors, half using that ops genius recomended CIs (20s to 30s and M.83) and half with CI150 -180 and M.84.
Gather all concerned data (ZFW, avg winds, traffic, etcetc), compared.
Findings:
- the ONLY difference is that using the first we spent more time in the air.
ABSOLUTELY NO DIFFERENCE in fuel burn flying on the little faster mode...this for 777-300ER with GE115;
Looks like FMC predictions are way too optimistic or just simply INCORRECT...
Try it yourself at home if you have a chance!
Remember that it's all about real numbers, no empty guess.