GM is a modern farming practice that is horrendously underused, and GM can wipe out most of the disadvantages of intensive farming - GM crops can fix their own nitrogen and repel pests without pesticide. They can also be designed to grow well in 3rd world climates and contain extra nutrients.
While I do agree that the planet can sustain a much higher population, modern farming practices are causing problems and not necessarily sustainable in the long term. (http://en.wikipedia.org/wiki/Intensive_farming#Disadvantages)
Properly thought out food production that utilizes under-used resources (vast swaths in Russia for instance!) is the key.