The energy content of corn is about ~100 kWh per bushel. If you can magically convert all of the sun's solar radiation into corn with perfect efficiency (~2,500 kWh/m2 per year), you get 100,000 bushels of corn per acre. Compared to the actual efficiency of 176 bushels per acre, there's a lot of room for improvement. It might not be possible with anything similar to modern agriculture, but it's fun to think about.