I believe GM crops to be the most sustainable form of food production. One of the biggest challenges for farmers is the control of weeds. Weeds reduce the yield of the crop in the fields. GM crops provide such strong weed control that farmers have been able to remove tillage as a form of weed control in many instances. Prior to GM crops, most farmers would produce a crop in the same field for two or three years, then the weeds would be so abundant that the farmer would till the field regularly for an entire summer and not grow a crop that year in an effort to get better control of the weeds as the chemical options were limited in the 1970s and 1980s. With GM crops, farmers get very effective weed control, removing the need to summer fallow a field. This reduces soil erosion and increased moisture conservation, as well as reduces the greenhouse gas emissions from farming as the farmer makes fewer passes over his field presently. With strong adoption of zero-tillage farming, the sustainability of agriculture has increased substantially from GM crops.
I might suggest that the continued growth in adoption and number of acres planted to GM crops (using ISAAA data) would be evidence of positive feedback, given that more farmers are adopting GM crops and growing them on more acres.