I'm not sure I understand this. Solar power could power the US with a 100 mile square area, and it seems like 'perfectly ideal' biofuel production would be just as efficient. Of course, solar panels are around 30% efficient right now (perhaps more for solar-thermal, etc) while biofuels are at negative efficiency - but that's what science is for.
Why would a perfectly ideal biofuel production be just as efficient? Commercial electric cells are close to 20% efficient. From that form to useful work is typically over 90% efficient.
By contrast from what I'm reading on Wikipedia the reaction that drives photosynthesis has a theoretical max of 25%. But you only get that efficiency at 1/4 of normal sunlight levels during the day (the rest of the light has to be wasted). We're already well below solar. And then you've got to give some of the energy to the plant for growing. Then you lose energy in the chemical reactions that make a biofuel. And the biofuel goes into an engine. Every one of these steps has a maximum theoretical efficiency that is fairly low. (For instance a gasoline car engine's maximum efficiency is in the 25-30% range.)
If I remember correctly, Elon claimed that biofuel production had a theoretical max of about 0.5% of incident sunlight energy being converted in the end to useful work. (We're a long ways away from this maximum in practice today.)
