I have seen no increase in food received from completing levels in S1 with the food harvester active. I have tried multiple different levels, and compared food received to data from Barry Farmz Here. The food I get is exactly what I would have gotten without the harvester in every case.
Of course, it’s possible that SGG has nerfed the food per stage. Only way to be 100% sure is to wait for my Harvester to run out tomorrow and do a comparison then.